Wayne, how much driving do you do at high elevation in flat areas? I know that generally high elevation and flat are mutually exclusive, but I've been through southern New Mexico, like the whole Tucson - Deming - Las Cruces - White Sands drive. Other than a single very sharp mountain range dividing Las Cruces from White Sands, it seemed to be mile after mile of flat, 4000+ elevation terrain according to my GPS.

Since both total aero drag is lower and pumping losses are lower as elevation increases, the end result should be better fuel economy. Now the last two times I've done that drive was last year, and I was just going for minimum time, like 75-77mph the whole way, but I still got far better gas mileage in New Mexico at those speeds than I ever do when in the 1200' elevation of Arizona at those same speeds.

I'm not fully sure how to calculate the increase in MPG, but assuming that the majority of the fuel is consumed overcoming wind resistance, 4500' elevation should be about 85% thick air as at sea level. Part of the loss of any throttled motor is also due to generating vacuum in the manifold, and since the air is less dense, there should be lower loss since you're generating less relative vacuum for any given MAP/CFM. Unfortunately, since there's a lot more things that make up the total loss besides just aero and pumping loss, I'm not sure exactly how to make the direct comparison of elevation's effect on FE, independant of hills.