Not a bad idea. Since I got my Blackberry, I no longer ask for receipts, so when I enter my data, I have to make sure I look back at the fill up and read what I put in. But I typically know that I fill up at a certain amount when I pull in, and I typically remember how much the fill up cost me ($30 is easier to remember than 27.456).

On road trips where I usually fill up a few times (going through canada), I wait till my bank statements show the cost, I add em all up, add up all my miles, and add up all my gallons (3.8L-1gal). I have my own spreadsheet that I input all this into, it gives me price/gal for the trip, which is the figure I have to use for fuelly.

This suggestion would save me a step, and if propose going a step further in allowing a road trip type fuel up, where 5,000 miles is an acceptable value to put in for mileage, or 110 gallons, ect.

Another site, virginiagasprices.com has a fuel logbook that you can fill out. I prefer fuelly.com, obviously, as it has more features. But one neat thing is that they use javascript on the three entry fields (price/gallon, total price, gallons) so that after you enter two of them (any two) the third one is calculated automatically.

I find that to be pretty neat.