From your initial description I thought maybe you were comparing specific makes and models. This would be more difficult, because you'd have to obtain (and maintain) this data from somewhere, but it's probably doable.
In that case, you could help potential buyers decide between one vehicle purchase and another based on fuel costs (not that your app can't do that now--the users just have to get their data someplace else). Nice of me to expand your project's scope, eh? ;)
Luckily, getting automotive data is fairly simple as there are a few companies that provide it and images (and dealer inventory too) for a fee (of course).
I've thought about having a list of "popular vehicles" like Prius, TDI Jetta/Beetle, Mini Cooper, Honda Civic, Yaris, Smart Car, etc in a pull down that will prepopulate the vehicle data.
Also, I've been ruminating about a better ridesharing/carpooling site/app and this would be a good driver to it.
I was wondering if you could license it to others (Edmunds, etc.) but I guess it depends on them. Maybe with your industry experience you know something about that.
Anyway, good luck!