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).
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