Something simple that is something I would like is a way to print out a custom sticker/paper to stick on the back window.
Something that could be cut out of a normal page and says something like
Formatting is prob all messed up on this example but I was thinking of a 4x6 inch or so oval shaped design with that information in it. I think the MPG number should be very big and either blue or black to help visibility to attract attention to the sticker. I could stick it inside the back window instead of printing it on something for a sticker. This way I could just put a clear plastic holder in the back and swap it out every so often. It would also be very cheap to do it this way.
Of course the top could be the logo from the top of the page or something. Since it would be a low use item bandwidth would probably not be a problem. The only thing that looks like it needs added would be a short link to the member gaslog/garage. The rest of it could be done individually but it would probably get a lot more people using it if it was automatic. Click here for your graphic, it opens in a window by itself and you print it cut it out and stick it in the back window or wherever.
Just an idea, I know people want stickers but this would be a very customized sticker and probably attract a lot more attention. Or maybe make people mad at you cause you get such good mileage.