The myth about air/heat and gas!!!

[StealthSnake36]

its a fact with engines the more loads you put on it the more power it needs to use to work that load. the AC compressor clutch kickes on it adds a new load so more power is needed. you get more power with a higher a/f ratio, more air and more fuel. even though on highways around 45mph its in direct drive so its gets its ideal fuel economy.

[shorod]

Originally Posted by StealthSnake36

its a fact with engines the more loads you put on it the more power it needs to use to work that load. the AC compressor clutch kickes on it adds a new load so more power is needed. you get more power with a higher a/f ratio, more air and more fuel. even though on highways around 45mph its in direct drive so its gets its ideal fuel economy.

What you're not taking into account is aerodynamic load though. With windows down, there is a good chance the vehicle has a higher aerodynamic load. Whether or not that load is significant compared with the load from the A/C compressor will be vehicle dependent as well.

-Rod

[J-Ri]

In my car, above 50-ish MPH seems to be better with the air on. If you have a long flat stretch of road where you are, just get going with the windows down. Once you are at a speed (hold the gas pedal steady), roll the windows up and turn the air on. If your car gains 2 mph with the same throttle, then the A/C is more fuel efficient than having the windows down.

[KiwiBacon]

A car with a real-time MPG readout would show this really easily.
Unfortunatley none of my cars do, I have a scangauge II which will plug into two of them, but the cars are too dumb to talk to it.