So running AC reduces your gas mileage. By how much? Does it depend on

[raccoon]

your speed?

[ingac70]

1. How big your engine is...

AC tends to drag on smaller engines, while making little or no difference with larger ones.

[tridim]

I think that's just for constant speed highway driving.

Running your A/C full blast in the city, sitting idle at stop lights half the time will kill your mileage.

[NightWatcher]

2. but does not reduce gas mileage more than rolling windows down at high speeds

this and more from the "what I learned on Mythbusters" file

[jakem]

3. according to Mythbusters, running the A/C

is more energy efficient than opening the windows and blowing your drag coefficient...

[tridim]

6. Weren't they driving big honkin' SUV's?

I really wish they'd revisit this myth using smaller cars.

[jakem]

17. i thought i remembered a pretty normal sedan for that test...

:shrug:

[tridim]

19. They need to do a whole show on gas mileage improvement myths

They've done a few random segments on the subject in the past, but they've never done hypermilling tests. Make it a two show special. There are plenty of things to test.

[snooper2]

20. They did it using two identical

Ford Exploders :)

[proud2BlibKansan]

7. It cuts my mileage by quite a bit

Unless I am on the highway. When I am driving around town, I try not to run my A/C.

[Renew Deal]

10. The issue is with a hybrid

The gas engine shuts off when the car isn't moving and the battery is charged. If the A/C is on and the battery drains, the gas engine will turn on.

[ET Awful]

13. In my Prius, I get about the same mileage with or without AC

Getting around 46 average either way.

[spinbaby]

14. My milage in a Prius

I'm not sure what you're doing to get only 46 mpg. I average about 55. And AC does cost from 5 to 10 mpg, but only when it gets hot enough for it to really be running full blast--90 degrees or above.

[kmla]

4. On average, it cuts my mpg by about 3-4 mpg.

I average about 31 mpg in my '01 Dodge, but it drops to about 28 mpg when the AC is going.

I don't speed like I used to, so I'm not sure if it varies at a higher speed...

[Fumesucker]

9. I suspect that at higher speeds, the percentage loss from running the AC will be less..

Aero drag is more important at high speed, so rolling down the windows will cost you more the faster you go.

Your AC compressor cycles on and off, at higher speed it should spend less time cycling on and therefore probably uses about the same amount of energy as it would at lower speeds.

One point, keeping your AC set on "recirculate" or "max ac" reduces energy useage for two reasons, first is the fact that once you have the cabin air cooled, it takes less energy to keep it cool than it does to cool incoming warmer air from outside. The second reason is that, in humid climates, a lot of the energy used by your AC goes toward dehumidifying the air, once the cabin air is dehumidified the car will not only be more comfortable at a given temperature but energy useage due to removing humidity will drop.

[nickyt]

5. raccoon - I caught a gas "expert" on cnn a few weeks back. I had been

driving with my AC off, thinking I was saving major gas. Well, he said that running your AC uses about a mile per gallon of your fuel, but that driving without the AC on, with your windows rolled down, increases wind resistance, and also uses about up about a mile per gallon. Hence - may as well run your AC.
I can't prove his statements one way or the other, but that's what he said -

[liberal N proud]

8. I always understood it to be a wash

What you gain in reduced engine pull you lose to wind resistance.

[Lex]

11. Rolling your windows down reduces your gas mileage too

by making the car less aerodynamic.

Not sure what the offset is, all told.

[Warpy]

12. Mythbusters did that one

and found that gas mileage was minimally better at slow city speeds with the windows down and the AC off. The break even speed was between 35-40.

Mileage was much worse at high speeds with the windows down and AC off than it was with windows up and the AC running.

If I had a gas hog, I'd probably suffer with windows open and no AC when I drove in the city.

My car is a gas miser, so I can bask in my AC. Neener.

[Ichingcarpenter]

15. Bush says running your AC at home and in your car at the same time

Will increase his gas mileage.

[TreasonousBastard]

16. Depends on a lot of things...

not the least of which is how old you car is, and if you have auto climate control.

Newer cars cut the AC off when you accelerate, reducing the huge demand for fuel with both AC and acceleration-- I don't rmember when they started doing this. Auto climnate control often has the copmpressor running all or most of the time, and balancing the cold air with the heater to give you a comfortable cabin temp and humidity-- a lot of them you can't turn off.

Now, that Mythbusters episode was pure bullshit. A humongous, and un-aerodynamic, SUV with a V-8 is nothing like the kind of car most people who care about mileage actually drive. The monster SUV already has lousy aerodynamics, so rolling the windows down adds a small percentage of drag and the excess power needed to either overcome the drag or run the AC will be minimal.

A car with really good wind tunnel numbers with the windows up will really suffer extra drag with the windows down and mileage will be reduced drastically, but each car will suffer different numbers, so no general number will make any sense. What could be assumed is that running the blower with the windows up will be much more efficent that opening the windows, and cycling the AC on occasionally won't hurt mileage too much.

[jakem]

18. Interestingly, i head recently that washing your car helps mileage by a notable amount...

study may have been funded by the car wash association...


anyone else hear this?

[NoGOPZone]

21. I've heard that it will improve mileage

although not significantly. I've also heard that a good wax job will help mileage. The theory in both cases seems to be that the car will encounter less wind resistance. I don't know if this has ever been tested, though.

[tridim]

22. I've also pulled off any "extras" on my car that increase wind resitance

My Civic came with a bra and a sunroof wind deflector. Both are now gone and my mileage went up by ~2mpg.

[Xithras]

26. I pulled the crossbars off my Subaru roof rack and gained 1.5MPG.

n/t

[Xithras]

24. Washing and waxing does make a difference in low drag cars.

If you're daily ride is an SUV, your aerodynamics suck so badly that a little bit of dirt isn't going to matter. If, on the other hand, you have a smaller, low drag, efficient car, having a lot of dirt and dust can make a non-negligible impact on your mileage. Dirt on your car increases drag at the surface, which creates micro-vortexes between the slower moving surface air and the faster moving compressed air flowing over your vehicle. That makes the airflow "dirtier", increasing drag, and decreasing mileage.

How much it decreases it is the question. A perpetual dirt film on an otherwise aerodynamic car may cost you a fraction of an MPG. That may not sound like much, but if you drive a lot it can add up to a few extra gallons a month. With gas flirting with the $5 mark, that adds up.

If you drive down unpaved roads a bit and have a nice caking of mud and dirt on your car, your mileage will take an even bigger hit because of the larger aerodynamic disruption AND the added weight you're carrying around.

[tammywammy]

23. I live in Texas

I'll keep using the A/C

[Xithras]

25. Yep, I live in California.

An enclosed car with no A/C can achieve internal temps over 150 degrees on sunny summer days, and just last week we had a few days where our ambient outside temp topped 106 degrees. Those temperatures can kill without A/C.

What gets me here are the people who run around with their A/C blasting all the time. I'm a born and raised desert boy and don't break a sweat until the temps hit the low 90's, so I usually keep my A/C off until the temperatures get into that range. I'm constantly floored, though, by the hordes of people I see driving around with their windows up and A/C running when the temps are only in the mid-70's to mid-80's. That's comfortable!

[Fumesucker]

27. A sort of counterintutive thing about hearing loss is that it makes you more sensitve to noise..

The older I get and the more my hearing goes, the more I find myself bothered by wind noise. For that reason I keep the windows rolled up more than I would otherwise.

On my bike I wear a full coverage helmet and use earplugs so it's not so much of an issue.