Carl, I'd say 18 mpg with your setup is impressive. The 30.5 inch tires probably help a lot. Try driving at 55 for the sake of curiosity. Air drag (in a very simplistic explanation) generally follows a quadratic formula...meaning the square of the increase x 2 equals the increased drag amount. Or if you increased speed from 30 to 60 (2 times) your drag would increase by 4 times. The coefficient of drag of the AD truck is bad, by the way. Worse than you would expect. Years ago I calculated that each 5 miles per hour over 50 mph (an arbitrary number) will mean you are paying apx. 25 cents extra per gallon of gas. We have a Subaru Forester. I once drove 175 miles at 57 miles per hour and got 40 mpg. It is not a hybrid, just a plain 4 cylinder engine with the CV transmission.
