You will never get max efficiency driving faster than speed limit. And a higher chance to get pulled over by cops. The rated mileage is assuming the ideal road condition and driving within speed limits. The faster you go the wind resistance increases which will reduce mileage.The mileage they rate as 38 mpg is based upon driving slow, 55-65. I can get the 38 MPG driving like that. However I typically drive at 80 MPH otherwise I would get shot in CA for driving too slow. With the 80MPH I get 30-32 MPG per tank of gas.