how much gas does the air conditioner in the car really waste?

The Mythbusters took on the question of A/C versus windows down.

http://kwc.org/mythbusters/2004/11/mythbusters_boom_lift_catapult.html

if you have the windows down, the drag on the vehicle will basically cancel out the lack of a/c. a/c doesn’t make all that much difference in gas mileage. tell him to lay off the hard starts and stops, do the speed limit, and he will see more improvement that way.

There was a Mythbusters episode about this; someone grab asavage and let him answer.

ah, here we go: http://kwc.org/mythbusters/2004/11/mythbusters_boom_lift_catapult.html

“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.”

According to that show Myth Busters it does not save much at all. That is if your car is in good working condition. If it is older and the ac is in bad shape then a couple of miles per gallon. Here is what you do. Unload as much crap from your car as possible to lighten the load. Fill your tires to the recommended spec pressure. Change your air filter often. Those things will help a bunch.

It depends on where you drive and how fast, at high speed on freeways the air conditioner uses less than the drag from open windows. around town at slow speeds, if it isn’t raining, the open windows will save you 3 or 4 miles per gallon. With the AC off. In rain, it is worth it to be cool and dry with windows closed and AC on!