2013 Interstate 3500, full tanks, A/C running continually, from Fort Worth, TX 626 feet above sea level, 740 miles to Chama, NM, 7871 feet above sea level, uphill all the way mostly on secondary roads (I hate Interstates, they bore me to tears), cruise control set on speed limit (mostly 70 to 75 mph) 19.5 miles per gallon average.
Return trip, downhill all the way and on a slightly different route with more interstates and 4 lane highways, 18.5 miles per gallon. The only explanation I have for the lower mileage on the downhill trip is because of the Interstate and 4 lane highways, the cruise control was set on 75 mph much more of the time.
Make errors — otherwise the Great Spirit realizes
you have finished your purpose on earth.
— Navajo saying