My car is brand new (I've had it for 2 weeks), so I don't have endless back data of gas mileage to enter. I find I keep wanting to fuel-up the tank (even after only ~80km) just so I have something to report! ;-)
The down side to having lots of old data to add is that you see how high fuel costs have risen. In less than two years, I (along with everyone else) am paying double for a gallon of gas. Guess I should have never looked back at the old data.... The up side is that I sure can see an improvement in mileage when I can use the cruise control and not hit the break for a couple of hours!