You're not alone. We've heard from a lot of people that they'd like Fuelly to move from being a fuel-tracking application to a total cost ownership application. Our most recent discussion of this topic was here and we haven't changed our minds on this one. We're going to stay focused on fuel economy.
For now you can enter maintenance into Fuelly as notes. (Click the Add a note link on the left menu.) But we're not ready to change the nature of the site by tracking expenses that aren't related to fuel economy.
It does seem like there would be an audience for that type of site. If anyone knows of some good total cost of ownership applications it'd be nice to have a place to send people when they make this request.