I have disagreed with Imped on this before. My experience with tons of tests using the scan gauge is that lower rpms will get better mpg in most cases. This is once you are up to speed of course. My tests would be as simple as watching the mpg readout, and then shifting up or down. Full throttle in 5th at 2200rpms is less than 1/2 or 3/4 throttle in 4th. It was never big, normally on the order of 1.5 mpg. Shifting at 2000 will kill your mpg.