How to add a brand new vehicle that wasn't filled up at 0 miles?
 

So I want to add my new truck, which gave me a free tank of gas when the truck had 7 miles on it.

How would I add it into Fuelly without having to worry about that random 7 miles? the truck was empty at 7 miles, and got filled up.

When you log your fill, subtract 7 from the miles driven.

so I won't have to subtract 7 miles every single time I fill it up?

I apologize, I fired that answer off incompletely. For the "Track fuel-ups with trip miles/km or odometer reading?" setting, which way do you have it set? I prefer to track with trip distance, not odometer reading, and I didn't stop to think that the other option exists.

If you track with trip distance then there's nothing more you need to do. If you track by odometer reading, I'm not entirely sure.

Fuelly does not do a fuel calculation for the first fillup for this very reason. Enter a fillup, and 7 miles as the odometer reading. Fuelly will start calculating mileage with the second fillup.