Even if it did happen, it would only drop highway truck consumption by ~5% (going from 75 to 55) and less freight would be delivered.

Five percent seems way too low for the speed change. Got any actual data?

This article says going from 65 to 75mpg raised fuel consumption
by 27%!

http://www.greencarcongress.com/2008/05/us-trucking-ind.html

This article says going from 65 to 75mpg raised fuel consumption by 27%!

That's probably true in general. Air resistance increases roughly as the square of speed, and so is 33% higher at 75mph than 65mph.

Wyoming, above, suggested a 20% reduction in consumption from the 30% lower wind resistance at 55mph vs. 65mph, which also seems reasonable, as one would expect non-wind factors to be larger at lower speeds.

Either way, though, there are quite substantial fuel savings to be had from reducing speed a modest amount.