All cars are less efficient at higher speeds because wind resistance increases with the cube of speed, and there are no efficiency benefits to travelling at a h

All cars are less efficient at higher speeds because wind resistance increases with the cube of speed, and there are no efficiency benefits to travelling at a higher speed. Driving at 65mph is substantially more efficient than driving at 70mph. In theory, driving at a constant 40mph would be more efficient still, but there are not too many roads where you can drive at that speed over a long distance without hitting a lot of stop lights or stop signs.