Interestingly, the movement for a lower national speed limit is being led by large trucking companies. "Con-Way Freight, one of the nation's largest trucking firms with 8,500 rigs, has announced it is turning back the electronic speed limiters in its entire fleet from 65 miles per hour to 62 mph." Frankly, I didn't know these truck fleets had speed limiters installed on their vehicles.
The reason for the change is not concern about the environment or concerns about safety; it's purely a financial decision, reflecting the higher costs of fuel and the lower mileage efficiency incurred at higher speeds (I can vouch for the latter, having tracked my mpg for years on trips). If the truckers slow down, they want autos to slow down as well, arguing that it is speed differentials, rather than absolute speeds, that cause most problems in interstate highway environments.
There is, as expected, controversy on this call for lower speed limits, at present principally from smaller trucking firms and independent truckers. The arguments are well delineated in the linked article and the accompanying commentary.