Currently I track my car's mpg with a google form + spreadsheet but it would be awesome to do it in beeminder.
The thing I think is missing is that beeminder would need to do math. I input at every fillup: odometer, gallons added, and cost (and date, assumed from form input date). This lets me calculate mpg since the last fillup as well as dollars per mile.
Considering you can actually do a lot in personal driving habits and car maintenance to contribute to additional mpg, so long as you aren't stuck in traffic often, it is actually really important to track how your driving behaviors have real money impacts in the form of mpg/dpm.
It would be totally awesome to add additional incentive to drive more efficiently on top of measuring.
Thanks for reading!