Saving gas in neutral?

I dont know if this is the right thread....

I just heard from a friend that you could save gas if you get up to cruising speed and switch into neutral and just coast. Then when you start losing speed he switches into 1 (I really dont know what any of the number are for besideds drive and overdrive I dont know what 1 or 2 are for) and drives with that.

Sounds like it would work but I just wanted your opinion on it?

In a word, no. It takes a certain amount of energy to maintain speed, a certain amount to accelerate, and a certain amount just to idle. If you drive normally, you are expending energy maintaining speed. If you do what your friend suggests, you expend energy getting up to speed, waste that energy by letting it coast, then expend energy again getting back up to speed. Not to mention; alterations in speed are the single largest contributor to highway deaths; second only to alcohol. Keep a steady speed, save gas, save lives. Period.

using cruise control would help it basicaly by what curtis has said

Thanks. I never actually tried it because my car is out of comission and now I'm not going to try it.

So what are the other gears for on an automatic transmission do any of them help you use less gas?

Nope. Put it in the highest gear, use a light right foot, avoid rapid changes in accelerator position, and slow down a few mph.

The only place I can think that will work is in mountainous terrain, where you might be able to coast downhill for awhile.

(However may not be a good idea on twisty roads or extremely steep descents, where you would want to maintain some engine braking)

Actually the gear selection you are using now is the best "p" !!!

Actually the gear selection you are using now is the best "p" !!!

:lol: How true.

I live in a mountainous/hilly area and I take it out of gear (manual) all the time to coast down substantial hills. Actually, I've passed cars in neutral in some areas where people doing the slow and steady and going absolutely nowhere fast. Timing is everything though. As Curtis stated, expending energy to get up to speed is worse. So I try to stay smooth and not coast real fast only to hit red lights etc. Honestly though, I've noticed a gain of a few mpg...

Driving around with the trans in neutral is illegal in most states. Also, using your cruise control works well on flat terrain but if you going up and down a lot of hills you are better off not using it.

i have seen posts here that say if you have an automatic transmission, that it is bad for the tranny to be in neutral when you are going at somewhat high speeds, due to lubrication issues or something. i have no idea how the tranny is lubricated or why you need to be in gear, but thats what the post said. i dunno if it also has anything to do with manual trannies or not

Jveik's right. Automatic trannys are designed to be operated in gear. The oil pump is driven directly from the engine and your pump's output is proportional to RPM. Putting the car in neutral lets RPMs drop to idle, so the pump is supplying the minimum lube for the moving parts of the tranny, but the tranny is spinning at 70 mph. Some trannys are fine that way, others will overheat or cause other damage.

Manuals are not as sensitive to it since they are simply bathed in oil. Since you're still turning some of the gears in neutral it still wraps lube up around the gears.

on decel most modern EFI systems shut off the injectors, so there is no fuel wasted. If you are on a steep hill and decelling in top gear maintains the speed you seak, then you will use less fuel leaving it in top gear and deceling, than you would putting it in neutral and coasting down the hill.

I agree with the above advice. Do not put it in neutral with an automatic. Saving 10 cents worth of gas isn't worth a $2000 tranny rebuild.

Is it bad to put it in neutral when you're at a stop light?

stop light no....while moving....yes. ive been doing this for abiout a year now. everything is fine. just made sure to keep it in gear on hikllks..

Bob your saying that most EFI systems shut off the fuel injectors yet the eninge keeps running. Did they design the engine to run on air.

Chiquae i love the sig you have to love stupid people.

stop light no other than the shifting in and out of gears which is where the most wear occurs in any transmission.

Bob your saying that most EFI systems shut off the fuel injectors yet the eninge keeps running. Did they design the engine to run on air.

Chiquae i love the sig you have to love stupid people.

I like stupid people too :screwy:

Yes, an engine will easily run with no fuel. But its a secwet....I can't tell you how they do it.

thanks about the sig. i hope u like that i put the picture of a shirt back in it. erone must buy 1 now!!!!

and love stupid people.... :grin:

I don't put the car in neutral at lights or signs. Seems like alot of wear and tear on the transmission for nothing. Going down steep hills, I'll usually leave it in the highest gear it will shift into - when I'm coasting down a hill in gear, the torque converter disengages (I'm assuming) and the engine goes down to about 1000rpm. I don't shift down becuase a set of brakes is much cheaper than a torque converter and a transmission rebuild. Manual I'll shift down somewhat - just keep the engine around 2000 rpm. I'm not going to make the engine go 4000 rpm just to slow me down.

Bob your saying that most EFI systems shut off the fuel injectors yet the eninge keeps running. Did they design the engine to run on air.

Chiquae i love the sig you have to love stupid people.

Most cars with electronic fuel injection shut off the fuel injectors under certain conditions. Usually the conditions are no throttle and an engine speed above a certain rpm (like 1500 rpm for example). Some might also have a gear limit, hence no fuel shut off in first gear for example.

So, going downhill you will save fuel by keeping the car in a high gear with no throttle. If you put the car in neutral the engine will consume fuel to keep it at idle. In gear, with injectors shut off the engine will be driven by the car slowing it down without consuming fuel, so you will saving your brakes too.

Going downhill in neutral will actually waste fuel, not save fuel!

Well i can understand under ceratan conditions, but at idle i doubt it. They have electronically regulated fuel injectors so at different rpm ranges the fuel injectors spray at different capacities. Yeah they will shut off in certain conditions and i know 5 speeds run completly different from automatics, And this might save at the most 1 mpg. Just dont accelerate hard, or drive way above the speed limit and your car should get close to the epa emissions sheet for your car.

To bob i was talking about his sig it said "my friends rx8 gets bad gas mileage but i guess thats normal for a v6" think about it.

Well i can understand under ceratan conditions, but at idle i doubt it. They have electronically regulated fuel injectors so at different rpm ranges the fuel injectors spray at different capacities. Yeah they will shut off in certain conditions and i know 5 speeds run completly different from automatics, And this might save at the most 1 mpg. Just dont accelerate hard, or drive way above the speed limit and your car should get close to the epa emissions sheet for your car.

To bob i was talking about his sig it said "my friends rx8 gets bad gas mileage but i guess thats normal for a v6" think about it.

At idle the injectors will inject fuel; but that's why you shouldn't put in neutral and idle. If you go off the throttle completely with a gear in, and the engine speed is over a certain rpm they will shut off.

If you want to save fuel you should also accelerate hard up to speed, then select the highest gear and keep the speed constant. This since the engine is most efficient at high load.

How you drive can save about 20% fuel, that is quite a lot!

Yeah that is alot, when i dont accelerate hard or speed alot i can get about 400+ miles a tank out of my blazer, if i drive the piss out of it i might get 150 to 3/4 of a tank its pretty shitty but that thing has a shit load of low end torque.

The farther down your foor goes, the lower gas mileage you will get.

OK yoda. But yeah thats the basics.

In a word, no. It takes a certain amount of energy to maintain speed, a certain amount to accelerate, and a certain amount just to idle. If you drive normally, you are expending energy maintaining speed. If you do what your friend suggests, you expend energy getting up to speed, waste that energy by letting it coast, then expend energy again getting back up to speed. Not to mention; alterations in speed are the single largest contributor to highway deaths; second only to alcohol. Keep a steady speed, save gas, save lives. Period.
Good post. Only thing I'd add is that it take more energy to accelerate, even at a slow rate over say 1mi than it would be to maintain a constant speed...this means if you have hills, the same speed up the hill as going down them.

If you have hills, the same speed up the hill as going down them.
NO!!! if you are driving 35 on flat ground and all the sudden there is a hill, leave your foot in the same place to go up the hill. yoor speed will fall about 3-5 MPH depending on your engines torque. at the top of the hill, lift off the gas and let gravity accelerate you back to 35. you usually wont have to use much gas to accelerate downhill, unless you are going like 85-90 MPH, in which case, your gas mileage is shit anyway.

see my sig, thats my car. when i accelerate hard and drive fast i get 8-10 MPG. when i coast a lot and let gravity accelerate me down hill and cut the engine at stoplights, i get 15-18 MPG. think about it, and it'll make sense

If you want to save fuel you should also accelerate hard up to speed, then select the highest gear and keep the speed constant. This since the engine is most efficient at high load.
most efficient yes, but most economical no. to accelerate hard, you push the gas pedal way down which lets the injectors spray as much gas as they can. how can this possibly be better than spraying just as much as you need to keep your speed.

NO!!! if you are driving 35 on flat ground and all the sudden there is a hill, leave your foot in the same place to go up the hill. yoor speed will fall about 3-5 MPH depending on your engines torque. at the top of the hill, lift off the gas and let gravity accelerate you back to 35. you usually wont have to use much gas to accelerate downhill, unless you are going like 85-90 MPH, in which case, your gas mileage is shit anyway.

see my sig, thats my car. when i accelerate hard and drive fast i get 8-10 MPG. when i coast a lot and let gravity accelerate me down hill and cut the engine at stoplights, i get 15-18 MPG. think about it, and it'll make sense


most efficient yes, but most economical no. to accelerate hard, you push the gas pedal way down which lets the injectors spray as much gas as they can. how can this possibly be better than spraying just as much as you need to keep your speed.

Keeping a steady throttle position will definitely save gas. And accellerating a tad harder might give better gas mileage only beucase the transmission might shift lower. And I don't mean floor it either. With my car 01 Taurus, the transmission won't shift until 2500 rpm if I don't push the gas that hard. A little harder (I'm only talking like down a centimeter) it will shift at 1500 rpm.

I don't know if it was posted in this thread before, but in areas with stoplights, try to coast through the whole time ... yes people will pass you, but you aren't ever stopping. In my case, I go 125-20 becuase my car will be in 3rd gear and I'll be going along at idle speed.

Hmm, kinda interesting thread and some very good advise. Lots of the ideas here will work for getting better mpg than the average driver.

In keeping with the topic, the most mpg when going down hill is with trans disengaged and engine shut off. Conditions will vary of course with terrain and speed desired.

Here's some others that will ring true especially when clarified, but will suffice to say that "all else being equal"

Slowest speed gets best mpg
WOT gets best mpg
Never use brakes gets best mpg
Large gear gets best mpg

Here's some others that seem wrong , but can actually be right. Letting off throttle when climbing hills can increase mpg. Pushing down throttle when going down hills can increase mpg. Accelerating fast can increase mpg. Decelerating very slowly can increase mpg. Varying the throttle can increase mpg.

It seems I have stated a lot of the opposites as some others, feel free to poke these ideas full of holes if you can,,, I know varying conditions and speeds can force things. I am imagining straight roads and lots of hills.

WOT does not give best gas mileage, and I thought the slower = better gas mileage was true, but it isn't ...

My car seems happiest at 70mph - 34mpg.

Size the engine with WOT and cylinders full will give better mpg than a "normal" or oversized engine that is part throttle even with it's advance and leaner ratio.

All else being equal a slower speed will give better mpg because of wind and resistance. Keeping in a big gear of course. As soon as the gear is changed to a lowr gear then the mpg drops. It goes right along with the above post to lug the engine so that it is gear bound and the thjrottle plates continue to open farther to maintain the same load. Not to go overboard of course to the point of chugging and sputtering.

The only way your going to get good MPG at WOT is with a briggs and straton on a go cart.

Aerodynamics, drag and any other effects on the cars MPG dont come into play until between 50 and 80mph. And at WOT your dumping the maximum amount of fuel and air into the engine, Even in short burst this will not get better gas mileage on ANY engine. Whether its a 2, 3, 4, 6 , or 8 cylinder at WOT that engine is moving as fast as it can go. You cant compare the gas mileage of a 5.7 to a 1.8. Even if they both have 400 hp its not comparable. That is like comparing an evo to a metro in terms of speed. Even then some of the v8 engines can get about 20-25 mpg. In my old honda i would get about 300 miles out of a tank if i ran it nice, if i drove the piss out of it i would be lucky to get 150. Its not comparable.

Each car is different as some one said, "My car seems happiest @ 70 - 34mpg." A Sentra I had was happiest @ 62mph - 52mpg. My Civic is happiest @ 83 - 38mpg. I hurd rumors of C5Z06's happiest @ 80mpg w/ 30mpg.

I've got a friend who's a mechanichal engineer. He broke down the basics for me of the factors that effect mileage.
1) Vectoring, primary factor but most forgotten. It's the mean between gravity (weight) and forward momentum (velocity). Too much velocity and the car comes off the ground and not enough and the engine has to work harder. Speed helps reduce the work needed to be done by the motor up to the point of wind resistance and drag become major factors.
2)Wind resistance. The force of air agianst the front of the car. Shape, size, and dirt play a factor here. A waxed car is more aerodynamic than an unwaxed though the speeds to have it make a noticable difference are usually in the triple digits.
3)Tires. Offroad tires will give resistance at higher speeds vs a Z rated tire that will give little.
4)Drag. Air's a fluid, as the car moves through it, a void forms behid it. This causes a vacuume that pulls the car backwards.
6)Engine set up/speed. Different engines are set up for low end torque, high end hp or a combination of both.
7)Gearing. Smaller gears aide in the transfer of torque to the ground while taller gears are better suited for aiding the in the transfer of hp to the ground. It's why a truck will have a "granny gear" for torque and pulling and the RSX-s has the 6th gear for top end speed and highway cruising.

i have seen posts here that say if you have an automatic transmission, that it is bad for the tranny to be in neutral when you are going at somewhat high speeds, due to lubrication issues or something. i have no idea how the tranny is lubricated or why you need to be in gear, but thats what the post said. i dunno if it also has anything to do with manual trannies or not I’ve read the same thing in other forums as well! Something about starving the engine or tranny of oil while you’re @ higher speeds because you’re @ a lower RPM.

Can anyone else with a lot more know-how than me explain this?:dunno:

the tranny is lubricated by the engine spinning it fast enough to spin oil up into the transmission. if you are coasting along as 75, and the tranny is spinning at idle, there is no way that it is getting enough lubrication.

for a while i started going extreme and not only shifting into neutral down hills, but i started shutting the car off down hill, if i had to stop at the bottom of the hill (yeah i know, its illegal and a bad idea). i did this for about a week untill i tried to shift back into gear after coasting downhill with the engine off and the engine flared for about two seconds then slammed into gear. i dont do it anymore and the tranny is hopefully ok.

to make a long story short, if you have an automatic, leave it in gear. you can shift out of gear when your car is stopped, but not while moving.
for a manual transmission, go ahead and shift into neutral all you want, it wont hurt a thing, just make sure you put it into neutral and take your foot off the clutch, or you will put way too much wear on the throwout bearing.

I'm going to expand on the tranny lubrication issue.

Manual transmissions get their lubrication via an oil BATH. There is a heavyweight oil in the transmission body. The gears are covered at least halfway by it. As they spin, they pick up the oil and transfer some to other gears directly. This will also splash some of the oil around and provide for additional lubrication.

Automatic transmissions have a pump that is driven by the engine spinning the torque converter. The faster the engine turns, the more faster the pump runs. This circulates more of the lubricating fluid through the transmission as it (the transmission) turns faster.

By putting the manual transmission in neutral, it's still spinning at whatever rpm the driveshaft (or cv shaft) turns it at. This provides ample lubrication supply.

The automatic transmission in neutral with the engine at idle only circulates the fluid at the level that it's at when it's turning at low rpm. By having the output shaft spinning at high speed without spinning the pump faster, you're not getting enough fluid circulation to meet the demand. Keeping it in gear keeps things connected enough to turn the pump fast enough to keep the transmissions internals lubricated.

The hydraulic pressure generated by the pump is also what governs how the transmission behaves. In modern computer controlled transmissions, that pressure is only half of the control story. But, it still does the bulk of the work of what actually happens in making the bits and pieces of the transmission do their job correctly.

This is only a simplification of part of the story of the automatic transmission, but it's enough for now. Take a look at howstuffworks.com and look up the automatic transmission. It's still a simplification, but a bit more detailed than what I put here.

howstuffworks is an amazing site. howstuffworks along with wikipedia, have pages of info on just about anything you would ever want to know.
i've been using them for a while now and learned so much. they are great sites.

alpurl, GreyGoose006; Thanks U guys! :thumbsup:

The only way your going to get good MPG at WOT is with a briggs and straton on a go cart.

Aerodynamics, drag and any other effects on the cars MPG dont come into play until between 50 and 80mph.

What SaabJohn and others are saying is correct.
Petrol engines are most efficient at close to wide open throttle.

This is almost solely due to the throttle. It works by strangling the engines air supply and it takes a lot of power to suck air past a closed throttle plate.
For this reason a smaller petrol engine running with wide open throttle will give better economy than a larger engine running with a closed throttle.

Try breathing through a straw for a few minutes to test the theory.

This is one of the advantages of a petrol-electric hybrid. The control system uses the electric motor/generator and batteries to run the smaller petrol engine as close to maximum efficency as it can.
This means at higher loads and wider throttle openings.

Of course an automatic transmission makes fuel saving a little harder.

Once again you CANNOT compare a smaller engine to a larger engine, its like comparing a mack truck to geo metro in terms of hauling power. ANd at WOT, you are dumping the maximum amount of air and fuel into the system. Thats why they have overdrive gears. In terms of power yes WOT is where efficiency comes into play, but in terms of gas mileage you are going to get mor mpg at 2000rpm doing 60 in 4th gear/5th gear than doing 60 at wot in 2nd/3rd gear.

The reason i say you cant compare engines of different class ie I4, v6, v8, I5, I6. Is becuase your taking one engien with say 2.0l of displacement, then say a 5.7l. Ones a 4cyl, and the other is an 8cyl. Let alone the fact that you have twice the amount of cylinders, you also have almost 3 times the displacement, so while it takes say 300cfm, mix this with a 14:1 air/fuel ratio for the 4 cylinder. Lets say it takes 650cfm with the same 14:1 a/f ratio. Ive seen some V8 engines get 20-25mpg and ive seen I4's get that and even into the range of 32-35mpg.

The throttle body isnt made to "strangle" the engine its made to control air flow at different engine rpm's. A closed TB isnt closed it has a small opening around the TB to allow enough air for the engine to idle correctly. And at WOT the throttle position sensor and a map or maf tell the engine how much fuel to mix into the engine, then the O2 sensor is a way for the engine to correct itself by checking the amount of oxygen/un burnt fuel to either add more fuel, or add less. So say what you want about a WOT 2.0 getting more mpg than a WOT 5.7 cause its not a way to compare engine MPG.

IF you came up with something like an idleing 2.0 gets better mpg than the same 2.0 at WOT than you are correct. But there is no way in hell that the same engine will get better mpg at wot than at idle.

ok, i dont know how this got so confused but here are the basics...
(all statments assume that you are using the same engine, and unless stated, are using it under the same conditions)

running an engine uses fuel
running an engine at a certain rpm uses a certain ammount of fuel
running an engine at a lower rpm uses less fuel and produces less power
to compensate for producing less power at a lower rpm, more throttle must be applied
when an engine produces the same power at a lower rpm it is running more efficiently
when an engine is running more efficiently it uses less gas.


the reason engines run more efficiently at lower rpms with more throttle is that there is less drag on the engine.
the major source of drag on an engine is the throttle plate. when it is closed there is so much drag on an engine that it produces verry little power and does it very in-efficiently.

think of a washing machine. when you want to wash your clothes, you save up until you have enough to fill the washing machine befor washing your clothes. you wouldnt run a wash if all you wanted was to wash a single pair of socks. it would be hugely in-efficient. an engine works the same way. it all has to do with loading. an engine gets its best mileage under a high load, at a low rpm, under full throttle.

this is why high horsepower engines use more gas. it is like having a washing machine running to wash a pair of socks.

while it is true that you cannot easily compare different types of engines, you can compare them to figure out why smaller engines get better gas mileage. consider the engine from a dodge viper. it makes 500 hp, and has 10 cylinders. now consider an old honda engine. they typically made 80 hp and had 4 cylinders. lets say it takes only 30 hp to drive 70 mph. in the viper, you are going to be driving at maybe 10% throttle. in the honda, you are going to be using something more like 85% throttle. the honda will get better mileage because the engine is sized more accuratley to the load required of it. why move 10 cylinders, when 2 is all thats required. why run a wash to wash a single pair of socks when you could save up until you could fill the washing machine and use it more efficiently.

I think it is pretty simple, 112 is keeping a constant (same engine) and it is true that wot will use more fuel, yet it will also do a lot more work and will have a far better bsfc.

If I was in a mpg race, I would be keeping a constant speed/load and then my WOT (small engine) would do better mpg than a large engine that was part throttle.

There are a whole lot more variables in engine design. The butterfly and advancing timing and lean mixtures are all good "bandaids" for trying to get the mpg back under a very light load. Car engines in my opinion are WAY oversized, and over powered for the job. If you had 2 vehicles the same size and weight and speed , the one with a tiny 3-4 cyl engine running nearly WOT will win the mpg race against the one with big v-8.

Once again you CANNOT compare a smaller engine to a larger engine, its like comparing a mack truck to geo metro in terms of hauling power. ANd at WOT, you are dumping the maximum amount of air and fuel into the system. Thats why they have overdrive gears. In terms of power yes WOT is where efficiency comes into play, but in terms of gas mileage you are going to get mor mpg at 2000rpm doing 60 in 4th gear/5th gear than doing 60 at wot in 2nd/3rd gear.

The reason i say you cant compare engines of different class ie I4, v6, v8, I5, I6. Is becuase your taking one engien with say 2.0l of displacement, then say a 5.7l. Ones a 4cyl, and the other is an 8cyl. Let alone the fact that you have twice the amount of cylinders, you also have almost 3 times the displacement, so while it takes say 300cfm, mix this with a 14:1 air/fuel ratio for the 4 cylinder. Lets say it takes 650cfm with the same 14:1 a/f ratio. Ive seen some V8 engines get 20-25mpg and ive seen I4's get that and even into the range of 32-35mpg.

The throttle body isnt made to "strangle" the engine its made to control air flow at different engine rpm's. A closed TB isnt closed it has a small opening around the TB to allow enough air for the engine to idle correctly. And at WOT the throttle position sensor and a map or maf tell the engine how much fuel to mix into the engine, then the O2 sensor is a way for the engine to correct itself by checking the amount of oxygen/un burnt fuel to either add more fuel, or add less. So say what you want about a WOT 2.0 getting more mpg than a WOT 5.7 cause its not a way to compare engine MPG.

IF you came up with something like an idleing 2.0 gets better mpg than the same 2.0 at WOT than you are correct. But there is no way in hell that the same engine will get better mpg at wot than at idle.

You have our claims confused.
I (and others) are saying that an engine geared to use WOT at cruise will give the best economy and efficiency.
For some reason you have read that as 2-3rd gear at WOT, which is very wrong.
Your comparison of idle to WOT is also completely unrelated to the previous posts.

I'm afraid you're confusing terms. So here's a primer.

Efficiency:
A measure of how much of the input energy (fuel) comes out as usable energy (shaft work).

BSFC:
Brake Specific Fuel Consumption:
A measure of engine efficiency, how many grams of fuel it takes to produce each kilowatt of output for one hour.

http://users.actrix.co.nz/dougal.ellen/bsfcmap.jpg

Throttling loss:
How much power it takes to suck against a vacuum. In this case it's the manifold vacuum created by the throttle.
For every revolution, your engine must pull the pistons down against the vacuum in the inlet manifold. The energy lost per stroke is the piston area*pressure difference*stroke.

Stoichiometric:
The weight ratio of air to fuel to acheive complete combustion. It is 14.7:1.

Air flow:
A four stroke engine can only consume half it's displacement of air per revolution. Lower revolutions consume less air and also less fuel.


It doesn't take much deduction at all to see that a lower engine speed results in lower pumping losses through less piston strokes.
At the same time, it is obvious that a more open throttle reduces pumping losses through a lower pressure difference.

But hey, don't take my word for it. Check out the graph above (if it loaded). Maximum efficiency (and minimum fuel consumption) occurs at 2500 rpm with the manifold pressure near it's maximum.

thanks for making that a little clearer. that is generally what i was trying to say, only, uh, better.
good info

I understand what you are talking about. However geared to use WOT will make good efficiency. But lower rpm's wll always net better MPG than any engine at WOT in any gear. Thats what im getting at this isnt the terms of brakeing efficiency compared to fuel consumption. Or the fact that a smaller engine at WOT will get better gas mileage than a larger one.

534BC Size the engine with WOT and cylinders full will give better mpg than a "normal" or oversized engine that is part throttle even with it's advance and leaner ratio.

WOT will tend to be used for acceleration, and passing at WOT your generally using higher rpms therefore using more fuel. Thats why driving style will determine gas mileage, if you push it to WOT from every stop light, accelerate at WOT, and keep it WOT till you get to crusing speed which is generally about 1500-2500rpm in OD. But in the same respect accelerate slowly letting the trans shift in between 3500-5500, keep the rpms low while driving on the highway you will get better gas mileage.

At WOT your efficiency is more or less judged in power to creat power you need something to create the energy, there for air/fuel, the more power you generate the more you will need to create the energy. I deal with drag cars that get gas mileage in the area of 1/4mile to 4-5 gallons. These cars are running constantly on WOT and will go through a tank of fuel in about 10secs. If you idle them they can run for hours, since theres no load on the engine it doesnt have to perform as much work.

I do understand what you are talking about but go drive your car and keep it at WOT all the time, then do it again and keep it about half throttle, you will get better gas mileage at half throttle than at WOT.

Im not worried about any equations, graphs its real world driving that will prove what works and what doesnt. I say this mainly because its the point ve been getting at the whole time, no 2 engines are the same.

At WOT your efficiency is more or less judged in power

No it's not, while power and efficiency are vaguely related, they are two very different measures.


I do understand what you are talking about but go drive your car and keep it at WOT all the time, then do it again and keep it about half throttle, you will get better gas mileage at half throttle than at WOT.


No I won't. Here are two scenarios.
1. I drive up a hill at WOT in 5th gear
2. I drive up the same hill in 3rd gear at the same speed using higher revs and lower throttle.

A petrol engine will be more efficient and use less fuel in scenario 1 than in scenario 2.

I think we can sum this up with "regardless to throttle amount, the lower the RPM's the better (within reason)"

I think we can sum this up with "regardless to throttle amount, the lower the RPM's the better (within reason)"
No, that's an oversimplification which completely ignores the whole kilowatts absorbed in throttling losses.

Take a look at the graph in my post above.
The engines efficiency can be doubled (or halved) at the same RPM by using the throttle.

hehe....you like arguing, don't you? :p

the less RPM's, the more likely you are to be using more throttle, so it fits quite nicely with your graph.

Which is a very cool graph btw, never seen that before

hehe....you like arguing, don't you? :p

Yes Mr Pot I do.

Regards
The Kettle

Damn i didnt think that posted.

I did just realize one thing i was talking more along the lines of an automatic trans. Since the trans will shift according to rpm.

In a manual setup i see what your talking about now. And in this setup i do agree.

I'll throw this in, pick it apart. It goes along with the topic, really.

Far better mpg can be attained by how we slow down than by how we accelerate. I am advocating to accelerate any way you want and as fast as you want, but to change the way you slow down.

Slow down "slowly" IE without the brakes and without ever applying reverse tork to the drive line. You will get a far bigger increase in mpg by always having forward tork applied to the driveline.

It is a bit hard to do at first, and sounds backwards, but it works.

Il agree it does work let the transmission do the initial work to slow it down then use the brakes to bring it to a stop.

Yes Mr Pot I do.

Regards
The Kettle

I like arguing specific views.....its a little different than your arguments I guess.

Since you didn't bother countering my argument, I will be nice and fill in some more on the logic I was using.

What it really comes down to is the speed you are going to cruise at in a given vehicle. If you never get over 5mph, then obviously it really doesn't matter what your RPM's are, because in all situations you will be barely above the idle stop on the throttle position. As you graph demonstrates so wonderfully, at extremely low throttle opening, there's not much noteworthy difference in effeciency.

As speeds increase, it takes more HP (KW) to maintain the speed. This is when the RPM matters, since the HP requirements will automatically require more throttle at lower RPM's.

The speeds required in the driving enviroment for each person, is more controlled by the area you drive, than a personal choice. Since you go too fast you lose your license, and if you go too slow, you get ran over, you are governed by the road you are on. This will have a large effect on your mileage.....no matter how you drive, for example, being stuck in stop and go traffic for 2 hours won't improve it.

The biggest factor you can manipulate when attempting to conserve fuel, is keep your speed down on the freeway. 55mph is a very efficient speed, and coupled with low RPM's, can pull in a huge increase....

But.....I never could drive/ride that slow. I may "only" get 45mpg on my bike when I ride at 80mph.....but I can live with it :iceslolan

Interesting point about slowing down slower. I've always done this. Why speed up to jam on your brakes at a redlight? Why put unnecessary wear and tear on your car? Sometimes, being SMOOTH is what makes all the difference. Accelerating smoothly can be beneficial in both a manual and an auto. You don't have to nail the throttle every time the light turns green... unless you drive a Honda... LOL... j/k... But really, by slipping the clutch, accelerating smoothly and shifting gears smoothly (take foot all the way off clutch before giving it gas) it helps. Same idea with cruising in neutral. While the other guys are flooring it to the redlight, I usually drop the car in neutral and coast slowly to the light. If the light turns green that's fine, I just smoothly put it back in gear. Of course, discretion is the key. If there isn't any traffic in front of me, I'm probably not going to neutral coast...

Some other factors to consider are aerodynamics (ie. what your car is shaped like and how big it is) as well as the terrain in which you're moving along plays into the whole how much work the vehicle actually needs to do. Basically, IMO driving smoothly is what saves gas. You don't have to drive 55mph to see big savings in fuel...

well i'm glad we came to an understanding here...
officially the answer is no. you cannot save gas by being in neutral (well you can but you will kill your *automatic* trans and the repairs will be far more expensive than the ammount of gas saved).