Some time ago friends of mine built the PetrolMonkey site. You can enter a basic profile/name for any car you own, and then input refills. The system then shows nice stats in both numbers and graph, and you can compare with other similar cars. There’s a minimal mobile version also.

By comparing Tiger (it’s the diesel growl ;-) with Otto (“the auto”), I already know that I’m spending about 40% less fuel now per km. Eat that, Kevin Rudd PM, with your pathetic 5% emission reduction over too many years proposal!