Claudia Bode at the University of Kansas and colleagues have put together an extensive, well-designed set of teaching materials to help students see the problems with "miles per gallon" and gain insights from "gallons per mile."
These materials are free to the public. They include both online pages and downloadable pdfs. The materials cover a wide range of topics on fuel efficiency, including the issue of driving speed.
Here are the two links to www.sciencecases.org:
Here is a link to teaching materials we prepared in early 2009.