Come to think of it; with the '93 Grand Voyager; if I inflated the tires to 44 psi cold and held the speed to 65 mph in the Intermountain Region I got 500mi to a tank of gas a couple of times and still had a gallon or two left. It seemed able to get 28 mpg at elevations above 5000 ft but rarely got over 24 mpg at elevations below 2000 ft. I suspect that the lower air pressure at higher altitudes was reflected by lower air density and less aerodynamic drag (15 percent or so would have done it). That and the lower-octane fuel sold in the mountains could have had a few more BTU per gallon of which the fuel injection system was able to take advantage. The tank nominally held 20 gal.