If there was a company that made an aftermarket guage, I would think there's a pretty large market for such a product. Especially as gas prices go higher.
It could be perfect for an info-mercial, too.

Eric Powers

MHG Organizer - www.MadisonHybridGroup.com
Hybridfest - A Hybrid Electric Car Show and More! www.hybridfest.com
Third weekend in July
Madison, WI