[MetroMPG]

One thing to remember is the relatively small increases in highway MPG from aero & gearing tweaks may actually work out to be better in real life, because the average speed of the EPA highway test is only 48 mph.

Yet another reason the official tests should include a simple top gear fuel consumption vs speed graph.

EG. if MPG @ a steady 70 MPH were a regularly discussed data point about cars, the manufacturers would have a greater incentive to do the mods (like aero) that would have a large effect on it.