Does having the AC in your car on lower your gas mileage?

Question

I recently saw something on TV saying that having the AC on in your car does not effect your gas mileage, having the windows down in your car will effect your gas mileage more. My husband swears that having the AC on will run down your gas. Does anyone know if this is true or not???

Answer 1

Are you amazed at the contradictory answers to what looks like a simple question? I like to use this reference when addressing this oft-asked question: In July of 2004 four GM engineers presented a paper at a Society of Automotive Engineers conference which addressed this question.
At much risk of oversimplification here are some highlights:
1. Below about 60-68F at between 31 and 68mph, using the A/C probably is more economical than rolled down windows.
2. Above about 68F fuel consumption with A/C on is always higher than rolled down windows regardless of vehicle speed.
3. Driving with A/C and windows up is the most economical choice.
4. Notwithstanding the above, individual results are dependent upon many other factors such as crosswinds and vehicle aerodynamics.

What your husband said is generally true.

Answer 2

Driving with the windows up and the AC off is the best combination of these factors to get the best mileage.

Driving with the windows up and the AC on will reduce the mileage some, but not a lot.

Driving with the AC off and the windows down will result in worse mileage than above.

Driving with the AC on and the windows open will be the worst combination for your mileage.

So your husband is right - it WILL reduce your mileage. But not as much as leaving the windows open. Tell him that if he won't run the AC when it's hot, you'll open the windows, and make his mileage that much worse.

Answer 3

Newer ACs run so efficently, that any reduction in mileage is insignificant. On older cars, especially with the smaller, 4 cylinder engines, a driver could actually feel the increased load when the AC was switched on.

If your car has a tachometer, you can tell how much load the AC adds to the engine. Leave the car in idle and watch the tachometer. Now, turn on the air conditioner. You may see a momentary "twitch" of the dial, but it promptly settles down to the exact same idle speed as before. That means that there is no additional load on the engine. No load, no decrease in perfomance or mileage.

Answer 4

If you run the A/C you will get lower gas milage. If you leave the windows open, you will increase air resistance and have lower gas mileage.
I had a full size Chevrolet Conversion Van. I tracked the mileage very carefully. I got 16 mpg in winter and 15 mpg in summer. Using the A/C was well worth it.

Answer 5

There is no truth in that statement... in fact if you drive you car down the road with your windows open without the AC on you will use far more gas then driving with your windows up and the AC on. this is because your car becomes a wind catcher and loses a lot of its aerodynamics from the windows being down. Also your cars engine is always turning the belt that runs your AC when the car is running... This theory was also shown on the t.v show myth busters on the discovery channel

quote from them:Windows down vs. air conditioning
"Urban puzzle": it is more efficient, on a hot day, to run with the A/C on and windows up than to run with windows down (b/c of increasing car's drag).

Computer-based mpg measurements:

11.7/11.8 with A/C on and windows up
11.7/11.8 with A/C off and windows up
11.3 with A/C off and windows down
So, according to the computer, it's better to use A/C with windows up.

This was too quick and easy for TV, so they decided to stage a seven hour marathon, race-til-you're-empty duel, with Jamie driving an SUV with A/C on and Adam driving an SUV with windows down. Though, once the safety inspector intervened, it was no longer a seven-hour marathon, it was a bit slower (45mph instead of 55mph), and a lot shorter (only 5 gallons each).

Jamie's A/C car ran out of gas first -- Adam's windows down SUV ran for another 30 laps -- completely contradicting the computer mpg estimate. Computer estimate based on air flow into the engine, so it would appear that it is unable to properly model the difference between A/C and windows down.

Answer 6

Actually, driving under 50 miles an hr. lowers your gas milage so it is best to keep the window open. If your driving on the highway, close the windows and use the a/c and you'll save gas because of less drag!!

Answer 7

Was it that episode of Myth Busters? I don't know. AC does use more gas than NOT using AC. As for the drag from having windows down, it seems to me they proved it well enough.

Answer 8

With your car in Park, turn on the AC. The engine noise will tell you. It takes more power, and thus more gas, to operate it.

Answer 9

who cares... consumer reports tested on a toyota andfound a loss of 1 mp gallon from 31 to 30...i paid 1200 dollars for AC in my truck im putting it on

Answer 10

There's a Mythbusters on this.

At least at 55mph (what I remember), windows down lowers mileage MORE.