There are three parts to this question:
- Does speed have an effect on miles-per-gallon? Is there a more accurate causal effect than speed that would also cause a connection at varying speeds? (perhaps RPM; weather; tire pressure)
- Are speed limits set at an appropriate point to directly affect miles-per-gallon? Should they be higher; lower? (Not to imply that gasoline usage is a relevant factor in speed limits.)
- Do drivers react to speed limits enough to matter? As in, do drivers obey speed limits enough to change their speed to one that improves their gas milage?
In addition, I would imagine that city and highway driving may have different results.