"Lowering your speed by 5 mph when traveling at 35 to 45 mph will boost fuel economy as much as 10 percent, according to [the GAO]."

At Wired's Autopia blog article entitled Slow Down and Spare the Planet:

Photo: Library of Congress. In Washington, D.C., speed limits went from 40 mph to 35 within 24 hours of the Baruch Rubber Investigating Committee recommending lower speeds nationwide in September, 1942.

It's worth clicking just for the photo if you don't have time to read the article and PDF.