I think my worst mileage ever was going from Phoenix to Tuscon. I was moving too as fast as I could in 5th without using any assist. I think I must have averaged 85mph if not closer to 90. AC was on econ the whole way set to about 78 with the fan manually set and it was very hot out and I don't recall feeling the compressor cycle off very much. I still got about 48mpg for that trip. I guess I whine a little about the extreme heat being a mileage killer when in fact I guess the extreme cold it worse.