I own a 08 Suzuki SX4 that I bought specifically to improve my gas consumption over my 98 Ford Ranger. As reguarding to AC vs windows open, I have noticed about a 2 mpg increase when I turn off the ac. My car has a meter in it that tell fuel econony, etc.
Think about it this way, when the ac clutch kicks in, it is a physical resistance on the engine. So now there is more work it has to do to keep the car moving, especially on the highway. You will notice this more with a smaller engine, mine being a 2 liter 4 cylinder.
I am making an educated guess that with a larger engine, for example a V8, there would be a less noticable decrease because of the size and power of the engine.
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