Using your AC does decrease your milage. I have always believed that. My mother and wife for years forced me to turn on the AC when its bearly hot hot. LOL
And my proof is, "Myth Busters" on the Discovery channel tested that Myth, that running your AC does not decrease MPG.
IT depends on your speed......
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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.
the MythBusters concluded that using AC was more efficient than opening your windows to cool down. However, they quickly admit that they were wrong to make such a hasty confirmation. In fact, they find that over 50 mph, you're better off with your AC on, but slower than that, open your windows.
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