The best my car has done is about 32 mpg going doen layfette,ind drove about 75-55 most of the way. My car is usually around 28-29 per tank. Yes most of the miles are highway on my car. On the way back I got about 25-26 80% of the trip was over 90mph. That highest was 95. My car didn't see to have much problem seem that it could do much much faster