There’s a new web-based application/service for the iPhone that allows you to keep track of the fuel efficiency of your car. It works quite simple and all it requires is that you start with a full tank of gas and every time you add an entry you should refill the tank. Kinda strange, but I guess it’s the only way to do that with the third party application, which is not connected to the car’s built-in computer system. Anyway, the Car Performance web app is free and you can try it out from here.