The main reason a lower speed limit cannot have a material effect on fuel consumption, besides being ignored by motorists, is that the preponderance of motor fuels is consumed on streets, roads, and highways that already have lower speed limits and, more importantly, lower speeds. Only 20 to 25 percent of all traffic volume is on highways with speed limits above 55 mph, and this traffic is already achieving superior fuel economy to that of traffic plodding along in urban and suburban areas. (Only 1.2 percent of the nation's 3.8 million roadway miles are interstate highway.) Add in conditions like congestion and bad weather where speed limits become even more irrelevant and it should become obvious that changing numbers on speed limit signs on roads where perhaps 15 percent of all fuel is consumed will not yield the nirvana of "energy independence."
If this country was serious about significantly reducing motor fuel consumption, it could start by redeploying the money being wasted on ticket-writing campaigns, laser guns, stealth cruisers, ticket cameras, and related wages and invest the savings in strategies to better move traffic in urban and suburban environments, where most fuel consumption actually occurs. There are huge savings to be realized by simply synchronizing and coordinating traffic signal systems. Cities that have started this process are not only reaping benefits like reduced fuel use, they are also realizing improved air quality, significantly faster commute times, far less congestion, and less wear and tear on vehicles. Removing obstacles to smooth traffic flow, including most stop signs and traffic "calming" devices, and scrapping other strategies intended to interrupt and disrupt traffic would dramatically improve fuel economy for the entire vehicle fleet.
I want to add that these strategies can be applied in a manner that actually improves the environment for pedestrians, bicyclists, and residential neighborhoods. We just need to get our collective heads out of the sand and rationally appraise our situation. Putting up arbitrary, irrational speed limit signs simply emulates the failures of the past.