Driving at a constant rate on the freeway with the windows up and AC on with the cruise on will obviously not have a huge impact on MPG.

But... if it's 120 degrees outside and you're in stop and go heavy, erratic traffic, I guarantee you're burning more fuel when you have the AC compressor running. Do your power bills go up in the summer time when the AC is running? Mine do!

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2001 X5 4.4i
2002 Honda S2000