Search | Car Forums | Gallery | Articles | Helper | AF 350Z | IgorSushko.com | Corporate |
| Latest | 0 Rplys |
|
Engineering/Technical Ask technical questions about cars. Do you know how a car engine works? |
Show Printable Version | Email this Page | Subscribe to this Thread |
|
Thread Tools |
02-06-2006, 05:16 PM | #1 | |
AF -Advisor
Join Date: May 2003
Location: Brick, New Jersey
Posts: 246
Thanks: 0
Thanked 0 Times in 0 Posts
|
Gas Mileage myths or facts?
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
__________________
Smoking is bad... especially blue smoke! Fill the oil and check the gas, please.
|
|
02-06-2006, 05:22 PM | #2 | |
AF -Advisor
Thread starter
Join Date: May 2003
Location: Brick, New Jersey
Posts: 246
Thanks: 0
Thanked 0 Times in 0 Posts
|
Re: Gas Mileage myths or facts?
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.
__________________
Smoking is bad... especially blue smoke! Fill the oil and check the gas, please.
|
|
02-06-2006, 05:24 PM | #3 | |
AF Fanatic
|
Re: Gas Mileage myths or facts?
^ i have often pondered these questions asswell, but i dont know the answer to either one.
|
|
02-06-2006, 05:31 PM | #4 | |||
AF Regular
Join Date: Jun 2003
Location: XCity
Posts: 347
Thanks: 0
Thanked 0 Times in 0 Posts
|
Re: Gas Mileage myths or facts?
Quote:
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. Quote:
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. |
|||
02-06-2006, 08:25 PM | #5 | |
Professional Ninja Killer
Join Date: Nov 2002
Location: Penn Hills, Pennsylvania
Posts: 3,561
Thanks: 0
Thanked 10 Times in 10 Posts
|
Re: Gas Mileage myths or facts?
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)
__________________
Dragging people kicking and screaming into the enlightenment. |
|
02-07-2006, 01:49 PM | #6 | |
AF Regular
|
Re: Gas Mileage myths or facts?
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.
|
|
02-07-2006, 04:42 PM | #7 | |
AF Regular
Join Date: Jul 2004
Location: Skamokawa, Washington
Posts: 259
Thanks: 0
Thanked 0 Times in 0 Posts
|
Re: Gas Mileage myths or facts?
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.
|
|
02-08-2006, 01:38 AM | #8 | |
AF -Advisor
Thread starter
Join Date: May 2003
Location: Brick, New Jersey
Posts: 246
Thanks: 0
Thanked 0 Times in 0 Posts
|
Re: Gas Mileage myths or facts?
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.
__________________
Smoking is bad... especially blue smoke! Fill the oil and check the gas, please.
|
|
02-08-2006, 10:30 AM | #9 | |
AF Moderator
Join Date: Jan 2005
Location: El Paso, Texas
Posts: 18,017
Thanks: 30
Thanked 53 Times in 41 Posts
|
Re: Gas Mileage myths or facts?
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.
__________________
'08 Pontiac Grand Prix GXP (Dark Slate Metallic) - LS4 5.3L V8 '02 Oldsmobile Alero GL2 - LA1 3400 V6 '99 Buick Regal LS - L36 Series II 3800 V6 '03 Honda CR250R MX - 2 Stroke 250cc '97 Pontiac Grand Prix GTP - L67 Series II 3800 V6 Supercharged (Sold) Timeslip 08/12/06 AF Community Guidelines |
|
02-08-2006, 08:55 PM | #10 | ||
Nothing scares me anymore
Join Date: Dec 2003
Location: City of Light
Posts: 10,702
Thanks: 12
Thanked 82 Times in 77 Posts
|
Re: Gas Mileage myths or facts?
Quote:
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. |
||
02-08-2006, 09:49 PM | #11 | |
Professional Ninja Killer
Join Date: Nov 2002
Location: Penn Hills, Pennsylvania
Posts: 3,561
Thanks: 0
Thanked 10 Times in 10 Posts
|
Re: Gas Mileage myths or facts?
Or you could be like me and run the AC with the windows down
__________________
Dragging people kicking and screaming into the enlightenment. |
|
02-08-2006, 10:02 PM | #12 | |
AF Regular
|
Re: Gas Mileage myths or facts?
Well the AC is broken in my car so I don't have to make a decision
|
|
02-09-2006, 08:48 PM | #13 | ||
AF Enthusiast
Join Date: Nov 2001
Location: Borlänge
Posts: 1,098
Thanks: 0
Thanked 0 Times in 0 Posts
|
Re: Gas Mileage myths or facts?
Quote:
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. |
||
02-10-2006, 07:34 AM | #14 | |
AF Regular
Join Date: Sep 2005
Location: raleigh, North Carolina
Posts: 282
Thanks: 0
Thanked 0 Times in 0 Posts
|
Re: Gas Mileage myths or facts?
jesus... just when you thought a post was dead and no more meaningless words could be spoken
|
|
02-10-2006, 02:02 PM | #15 | ||
AF Regular
|
Re: Gas Mileage myths or facts?
Quote:
|
||
|
POST REPLY TO THIS THREAD |
|
|