One point: we today move more of our goods through over the road trucks than we did 30 years ago when this stupid idea was actually enacted.
At 55 mph the movement of these goods would take longer and lengthen the inventory cycle causing costs to increase. It would also limit the number of miles per day a truck could travel between 200-250 on average. This raises the cost per mile and lowers the owner/operators profits.
I bet the Teamsters would love that!
Reducing the speed limit also reduces the through-put of the highway. When the freeway through-put decreases, the car density increases. If the car density exceeds the carrying capacity of the highway, you have stop and go traffic. Then the gas mileage drops down below 20 MPG. When traffic flows through a highway system in a stop and go pattern, the accident rate increases.
One way to increase fuel efficiency is to drive in the "sweat spot" of speed-to-mileage. Such a sweat spot is nearly impossible for the average person to calculate. However, for about $170 one could install a electronic device that would give your instantaneous MPG. Such would give one the type of feedback they need to determine what type of driving habits would optimize their auto fuel economy. Each auto has a different optimum speed. For one to determine their car's operating performance one needs the ScanGaugeII at:
http://www.scangauge.com/
Another method is for manufacturers to start making performance curves for each brand of automobile. That way drivers could pattern their driving habits for their vehicle's performance characteristics.
One thing for certain, we do not need politicians to tell us what the optimum speed should be for our automobiles.
One also needs to realize than for every 100 lbs of extra weight their car is carrying, MPG is reduced by up to 2%. This is something to consider when one is tempted to use their trunk for extra home storage space or neglecting to pull out that bag of cement you purchased a week ago.