Slightly adjacent to your question, but I've thought it would be nice to have a Google Maps-like app that optimizes for fuel use instead of drive time or distance.
If there are any Googlers reading: I'd love if Maps implemented this. Helpful for saving people some cash, too, which I am sure would be popular! As it is I have to consider distance, time spent, hilliness, speed limits (to limit wind resistance), and amount of stop & go, then guesstimate.