As areodynamics and fuel efficiency improve should we continue to slow down?
Perhaps you misunderstand me; I'm no more in favor of the 55 mph speed limit than you are. If it were up to me, I'd say 80 or better is about right in non-congested interstates....
See my earlier post in which I address the issue of how speed limits are properly established i.e., by the 85th percentile rule. IOW, speed limits, to the extent that there are any, ought to be established based on safety, not fuel consumption issues.