I'll definitively give that a go. Do you use the mpg reported by the computer or do you calculate it?
I personally use my trip mileage when I text them the information. They give you the option of using your total mileage as well, but if you happen to forget to log a fuel up then it gets a little quirky for the next time around. So no, nothing on that site is based on the MPG computer, but the trip computer.
As an example, when I text them my text will usually include the following information in it: "330.4 3.379 9.440"
You don't include the quotation marks of course, and the numbers signify miles, price per gallon, and amount of gallons put in respectively. If you read their site a little bit then you can actually add some more information to your log via text like the amount of city vs. highway driving you think the trip was, and additional notes. I just text the above info because it is easier. Only one number comes off the vehicle as you can see, so I usually text this all as I'm sitting at the pump. Its become a slight habbit so I haven't missed a fuel up yet.
****ing Focus (Ford Focus) | Fuelly
That is my current statistics for my Ford Focus. Don't mind the name of my vehicle, but this just gives you an idea of some of the stats you get when you have logged a few miles. I bought the car new, and started the Fuelly account after one fill up I believe so this is about 300~ miles shy of the actual mileage on my car. I wanted to compare it next to Ford's claim of 36MPG highway and I'd say I'm doing well. My commute is like 90% highway.