I'd like to be able to enter Miles and Gallons - that way, you could ensure that users are giving you real data instead of the display readout. Let the server calculate the mpg. In my own table I add the display mpg and calc the pct error, because I'm a dork like that.

It might also be neat to add an optional price paid or price/gal, or find an RSS server that updates the nationwide average, and display the actual hybrid savings on the graph next to the ugly "nonhybrid average mpg" bar!