“Slow Down and Save Gas” says CNN
Here is an article on CNN that talks about how you can save gas by driving at optimum speed.
I have a similar post, but my argument was based on the gear train efficiency. This article talks about air resistance. As noted there, the resistance of air increases exponentially as the speed increases. So obviously if you are driving slower, the energy your car spends in overcoming air resistance will be significantly less. As your speed increases, this fraction increases rapidly.
This will also help explain why drafting behind a vehicle will result in fuel saving. Also this must be a factor that makes cars more fuel efficient compared to SUVs.