The emissions and efficiency difference between 55, 60, 65, and 70mph are significant and cannot be understated. It makes such a real difference for air quality that TEXAS (the state that hates regulation and built the monstrosity called the Katy freeway) has a reduced speed limit in some metro areas for air quality reasons.

In fact, the reason most national highways have a speedlimit of 50mph, even out in the boonies in Kansas where everything is flat and straight, is because of the fuel crisis of the 70s.

When I was a child, I was curious why highways were 50mph but the interstates had a speedlimit of 65-70mph, so I went and found out instead of assuming it was a disingenous revenue grab.

Most interstates were generally built after the fuel crisis, and they modified the national speed limit in 1988, setting the speed to 65mph, again for efficiency reasons. It was repealed in 1995. Perhaps 65mph is what was considered safe for the road and vehicular technology at the time, and no one has had the political or municipal capital to do a new study ever since.

You want that speed limit changed, contact your reps, I guess.