I have notice that our guzzlers-Pilot-Titan get better mpg in "mountain" areas.They got 24.1 and 21.4 mpg from DFW-Flagstaff AZ despite AC blasting.The New Orleans leg to Dallas was much lower,but it was bumper to bumper traffic and not representative.
I'm pretty sure it is the decreased drag-lower atm pressure-that improves our "western" mpg.Despite the hills which are usually poison for non regen braking vehicles,we always do well out west.One tank(for both vehicles) was particularily good.The Albuquerque to Gallop leg probably averaged ~4500 feet.The Pilot got 28 mpg,the Titan(big V-8 full sized pickup) got 24 mpg-sort of an informal record at TITANTALK-a very active Titan forum.The wind was blowing very,very hard on that leg,but I was so tired that I didn't note the direction(probably had some tailwind component).
My point is the obvious-and no doubt stated here very often,but I am new here-altitude helps mpg-a lot!!The 21.4 mpg over 965 miles is very good for a guzzler like the Titan.I do have a chin spoiler than MIGHT-MIGHT- have cleaned up the air a little-the underside of pickups are very cluttered-and I have wheels that are 11 lbs less than stock,so they certainly help in any slow down speed up driving(there were a lot of work zones which involved dropping speed to 45-55 miles per hour-this lower speed(otherwise 68 miles per hour) helped the mpg.
Folks on the high plains could post the best mpg.They have a huge advantage.Thanks.Charlie