During World War II, there was both gasoline rationing and a national 35 mph speed limit. Of course, other than a few parkways in New York, a few freeways in California, and the Pennsylvania Turnpike, there were no high speed limited access highways in America.
WWII gas rationing and the speed limit were intended to save rubber rather than fuel. Once the Japs took over Malaysia, much of our rubber supply was cut off.