*please see below for results of a test I performed when the cabin temp was close to outside temp* Considering the recent discussion regarding MPG losses with the AC on I decided to play around with the AC while watching my ScangaugeII. Method: I started testing on the freeway. Speed was 65mph. I decided not to use CC due to major fluctuations in instantaneous MPG. I held my foot steady to maintain 65mph and chose the flattest section of freeway that I knew of. I would then turn the AC on with temperature set at 78degrees while monitoring the instantaneous MPG gauge onthe ScangaugeII. Then I would turn it off again while continuing to monitor the instantaneous MPG. Result: Every time I turned on the AC, the instantaneous MPG would drop by approximately 10mpg. I watched the MPG gauge drop from 72mpg to 62mpg. Sometimes it was not an even 10mpg drop and I would see numbers ranging from 8mpg-12mpg but this could simply be due to the changing road conditions. We all know how quickly the gauge jumps around. I then decided to take a back way home where the road is smooth, level, and I could go slower. I performed the same test but took video this time. I observed the same approx. 10mpg drop while maintaining a speed of about 58mph. I then tried turning down the fan setting to the lowest setting possible and nothing change. I continued to observe the 10mpg drop. After I finished taking video I rolled down the windows and the MPG stayed about the same as with the AC off. Summary: In the short bit of testing I performed I would almost be inclined to say that AC use may have a much more drastic effect than many of us realize and having the windows down may be better than running the AC when traveling at lower speeds (0-60mph). I would love to see someone perform a much more robust test to see if the MPG loss is as bad as it appeared during my test. * With the temperature set close to the outside temp (64deg outside 68deg inside) I tried cycling the AC on and off like I did yesterday and I only observed a 1mpg change. The change was very consistent. This is along the lines of what bisco is experiencing.