Gas Mileage myths or facts?


ice745
02-06-2006, 06:16 PM
I've heard that both A/C and Overdrive bring down gas mileage.

Personally, I think Overdrive would help gas mileage, because (as far as I know, I may be wrong) it just allows the transmission to go into 4th gear. A 4th gear would help decrease RPM, and therefore help gas mileage? Am I wrong? I really don't know that much about what Overdrive does if not just allowing the use of the 4th gear.

The other thing is with the A/C I don't understand how it affects gas milage. The compressor is belt driven, and its not like the compressor itself consumes gas. Since the belt is always going to spin whether the A/C is on or not, I don't see how it can affect gas mileage. I understand the pully will only spin if the A/C or Defroster is on, but does the amount of force needed to rotate a pully really affect gas mileage? Or is there something else I'm missing.

I may be wrong in both cases, but with what I know it doesn't seem to make sense, maybe I'm naive and missing something. If anyone can confirm if they are myths or facts, please give reasons along with it.

Thank you :)
Russ

ice745
02-06-2006, 06:22 PM
Apparently with the A/C its because of the electricity it pulls that affects the mileage. If you have a strong alternator, and a high performance battery, like Optima Yellowtop; will the A/C (along with other electic accessories) still have as much of an affect on gas.

G-man422
02-06-2006, 06:24 PM
^ i have often pondered these questions asswell, but i dont know the answer to either one.

amanichen
02-06-2006, 06:31 PM
I've heard that both A/C and Overdrive bring down gas mileage.

Personally, I think Overdrive would help gas mileage, because (as far as I know, I may be wrong) it just allows the transmission to go into 4th gear. A 4th gear would help decrease RPM, and therefore help gas mileage? Am I wrong? I really don't know that much about what Overdrive does if not just allowing the use of the 4th gear.Many modern cars (especially with automatic transmissions) are geared so that fuel efficiency at highway speeds becomes a priority. For instance, the first three gears on an automatic transmission might be closely spaced for acceleration at midrange speeds. The car is then programmed to tend to shift into 4th gear right around the normal speed limit (60-65 mph for most cars in the USA.) The 4th gear size is designed so that the RPMs are as low as possible, while the keeping the engine spinning fast enough to power the car along at 60mph.

For manual transmissions, this is of course different, because you can select your own shift points. However, the same gear-ratio concept still applies: at cruising speed, the gearing is selected to minimize engine RPM. Some cars with manual transmissions can have gearing which allows them to hit their top speed in 4th or 5th gear, where 5th or 6th gear is used for economy cruising.

The other thing is with the A/C I don't understand how it affects gas milage. The compressor is belt driven, and its not like the compressor itself consumes gas. Since the belt is always going to spin whether the A/C is on or not, I don't see how it can affect gas mileage. I understand the pully will only spin if the A/C or Defroster is on, but does the amount of force needed to rotate a pully really affect gas mileage? Or is there something else I'm missing.Anything in your car that uses power must get power from the engine. If you want to maintain a certain speed in your car after turning on your A/C system and 1000 Bagigawatt sound system, your engine must produce more power. Producing more power means more gas must go to the engine, hence, for every mile traveled, you're using more gas.

As for the actual impact of using A/C: in some cases, the gas mileage hit from A/C is smaller than rolling down the windows.

curtis73
02-06-2006, 09:25 PM
On the average, A/C for larger cars takes as much as 15 hp to run. That means you have to put that much more "foot" into it to maintain speed or to accelerate with the same force you're used to.

Overdrive does drop RPM, but lower RPMs aren't necessarily always better. Different engines have different peak cruise efficiency points. An overdrive on an already high-geared vehicle will cause the engine to lug down requiring more foot to maintain speed. For the most part, the manufacturers do a relatively decent job of finding a good cruise RPM, but sometimes they go a touch on the conservative side (RPMs too low)

cody_e
02-07-2006, 02:49 PM
A myth that I think is the cruise control myth. Supposedly if you are on the highway they say to use crusie control to maintain your speed. I might be able to see this on a flat surface but anywhere with hills I could see this being way mroe ineffecient. I mean haven't you ever turned the cruse off for a minute so you could turn and then for the hell of it press resume after the turn? You buy like a gallon of gas getting back up to speed.

kcg795
02-07-2006, 05:42 PM
I don't really see a difference in my mileage with the cruise control on or off. With the cruise off, I can often get a head start for a hill and be able to make the hill before the transmission will drop out of overdrive. If I can keep it in overdrive as much as possible, in theory, I save gas.

ice745
02-08-2006, 02:38 AM
Cruise control helps because it maintains speed. Where as a lazy foot would reach speed, let off the gas completely, and then accel again when the speed drops. It takes a lot less force to maintain speed then to accel. Cruise Control can be bad sometimes because it lacks the intelligence to slow down for bends and speed up before hills. It is also boring because it lacks the thrill-seeking to accel quickly for little bumps for that brief sensation of weightlessness. It also lacks responsibility to pay for speeding tickets! >:O

Cruise Control can waste gas if you "resume" it after slowing down. You should get your speed back up manually, then "resume" it. Cruise Control is just meant to maintain speeds, not neccessarily accelerate to them.

BNaylor
02-08-2006, 11:30 AM
Personally, I don't think it is a myth concerning the benefits of overdrive or even using the cruise control. Of course, it depends on the driving conditions and terrain. On a trip/highway my gas mileage improves 5 - 6 mpg. However, in city it is a different story. I'd rather be taching 2000 rpms cruising at 75 mph than 3,000 rpms. Torque convertor clutch (TCC) lockup helps out too. On use of A/C I get only a 1 mpg gallon difference in gas mileage with the AC off.



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MagicRat
02-08-2006, 09:55 PM
The other thing is with the A/C I don't understand how it affects gas milage. The compressor is belt driven, and its not like the compressor itself consumes gas. Since the belt is always going to spin whether the A/C is on or not, I don't see how it can affect gas mileage. I understand the pully will only spin if the A/C or Defroster is on, but does the amount of force needed to rotate a pully really affect gas mileage? Or is there something else I'm missing.


The difference here is the draw the AC system uses.

When you do not run the AC, the compressor pully spins around, but the compressor itself does not turn. It is mechanically disconnected from the pully.

When you turn the AC on , the compressor engages and rotates. This rotation draws power from the engine, so the engine must burn more fuel to move the car AND spin the compressor.

Using the AC on the highway is not always a bad thing. Often cars get better mileage using the AC and having the windows closed, than no AC and having windows open.
In this case, the extra aerodynamic drag of open windows is more than the drag of the compressor.

curtis73
02-08-2006, 10:49 PM
Or you could be like me and run the AC with the windows down :)

cody_e
02-08-2006, 11:02 PM
Well the AC is broken in my car so I don't have to make a decision ;)

SaabJohan
02-09-2006, 09:48 PM
On the average, A/C for larger cars takes as much as 15 hp to run. That means you have to put that much more "foot" into it to maintain speed or to accelerate with the same force you're used to.

Overdrive does drop RPM, but lower RPMs aren't necessarily always better. Different engines have different peak cruise efficiency points. An overdrive on an already high-geared vehicle will cause the engine to lug down requiring more foot to maintain speed. For the most part, the manufacturers do a relatively decent job of finding a good cruise RPM, but sometimes they go a touch on the conservative side (RPMs too low)

15 hp is a bit much, but up to 10 hp is possible. Mean power consumption is however much lower, perhaps a hp or two. With modern cars you also won't need to give more gas to compensate for this loss, that is done automaticly with the modern electronic throttles. The loss is however still present.
Older cars usually used to disconnect the AC compressor during high load (like acceleration), today this is usually solved by letting the engine produce a few more horsepowers instead.

A gasoline engine is more efficient at high loads (with the throttle more open), so a reduction in engine speed and an increase in produced torque (higher load) almost always result in a decreased fuel consumption. Even if it results in engine vibrations. The highest efficiency are usually found at about 70% of WOT, slightly less with a turbocharged engine. Load has a much larger effect on engine efficiency and specific fuel consumption than engine speed.

mazdatech177
02-10-2006, 08:34 AM
jesus... just when you thought a post was dead and no more meaningless words could be spoken

cody_e
02-10-2006, 03:02 PM
Older cars usually used to disconnect the AC compressor during high load (like acceleration), today this is usually solved by letting the engine produce a few more horsepowers instead.


Newer vehicles (at least 1998) stil do that. They let the comrpessor spin every 15 seconds or so to keep the bearings from going out in the compressor itself.

curtis73
02-10-2006, 04:28 PM
15 hp is a bit much, but up to 10 hp is possible.

You've evidently never done much work with Frigidaire A6 compressors :) 17 hp is common for some of their bigger ones, and some larger 70's American cars use TWO of them.

Moppie
02-10-2006, 04:46 PM
15 hp is a bit much, but up to 10 hp is possible.


In a small modern car yes, but I know of some early American cars that had compressors that needed as much as 70hp to drive them, thats more power than my 1st car produced.
Of course A/C technology has changed a lot since then :)

SaabJohan
02-19-2006, 07:36 PM
Newer vehicles (at least 1998) stil do that. They let the comrpessor spin every 15 seconds or so to keep the bearings from going out in the compressor itself.

Newer cars are equipped with electronic throttles and variable displacement A/C compressors. With the help of variable displacement the compressors can be connected to the engine most of the time the compressor is used. Heat removal is controled by changing the displacement. Power loss is compensated with help of the electronic throttle. The pressure in the A/C system is used to calculate how large the compensation must be. A variable displacement compressor isn't turned on and off so many timed but when it is, the electronic throttle and ignition timing is used to make the connection/disconnection smoother.

Older cars with constant displacement compressors tend to do turn the compressor on and off several times to control the heat removal. During full load they were usually turned off. No or some compensation was used. When compensation was used this was to allow for a smooth connection/disconnection.


You've evidently never done much work with Frigidaire A6 compressors 17 hp is common for some of their bigger ones, and some larger 70's American cars use TWO of them.

A modern compressor like a Sanden SD-7 series takes about 8 hp or less. This is enough for a large modern car.

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