On one long trip to California over the Sierra Nevadas I was keeping track of each tank. During one stretch I was doing 80 mph and got 30 mpg. So I slowed down to see if I could improve it. Not much difference. On this particular trip I got the following readings per tank. 30.46, 28.05, 29.05, 29.83, 29.45, 29, 25.93 and 30.03. It is the aerodynamics of the car that provides such good gas mileage. In town it goes down to 17-20 mpg.