This week Garmin announced EecoRouting for their 2X5 and 7X5 series of GPSs. You can choose your routing type. It is a free update.
http://www8.garmin.com/buzz/ecoroute/ Click on "Free Downloads".
Sadly I have the 200w and not the 205W or I would definately check this out. It even claims to give real time fuel mileage and other great information. Also click on the Conserving Resources tab. This could turn out to be really cool.
It seems like a good alternative for pre OBD-II cars. I would like to see how it compares with a scangauge. It can even calculate fuel burn for your route and give you the cost.