I just purchased a new (used) car. However, the tripometer was not set. I'd like to track my fuel purchases using Fuelly, but I'm not allowed to enter my first fill-up without entering the number of miles since the last fill-up. I know that I could track using the odometer but would prefer not to do that. Any help would be appreciated!
Yeah, those first fill ups can't count with a new (either off the lot or new to you used) cars. It's by design -- in the life of any car, you only get one first fill-up and it's not that big of a deal to skip it or set the tripometer for the next fill up, while all the other thousands of fill-ups over the car's lifetime will be easily recorded.
I was a tripometer type of guy when we designed fuelly, but I'd strongly suggest you use this first fillup to start using it with the Odometer. With the odometer tracking, you'll never have to remember/forget to zero out your tripometer, and when people borrow your car you won't have to explain how to reset the tripometer (though you do have to tell them to write down the mileage if they get gas).
When I got my car, I took it to a station and filled it to the brim. Then I drove 4000 miles and when I got home I had to wait for my card statements and added it all up, and had to divide it into like 4 segments to make fuelly happy.
But, disregard the first fill up ( should be like 1-2 gallons anyway) and take mileage from that point on.
I just faced this issue. I just filled-up and checked the box indicating I had missed a fill-up. It seemed to work fine. Something specific to deal with your first fill would make it more intuitive, though.