The middle of last year I uncovered the FuelFrog website which let’s you track your fuel mileage.
The MyMileMarker website serves the same function and let’s you track. MyMileMarker lets you collect all the information that’s important in tracking your mileage and travel information. They then provide you with some great reports of the information as well. You can also submit your fill-up information through their mobile site, or via twitter too.