It would be sweet to have a best and worst indicator along with the Average MPG on both the car page and the HTML code for pages/forums.
It would also be neat to be able to add a significant event such as a new type of tire, oil change, etc. show up on the fill up chart so that one could see a trend before and after a certain fill up, oil change etc.
Passing thought. An octane field and an ethanol checkbox that would also put a dot of a different color on the chart indicating a different octane and with/without ethanol indicator. And then add a readout for averages with different octanes side by side so one could make a comparison between them.