Yea i refuse to do an "initial" fill up, just messes up your other data, like average miles per tank, as 0 miles on your first fill up still gets included.
it took me several attempts to get my first fill-up after getting a new car to register. You would think that adding a note or even a service with the odometer reading when you drove off the lot would be propagated to the previous odometer reading when doing YOUR first fill up. No, I had to put in a bogus partial fill-up reflecting the original odometer reading to get a reasonable MPG reading on my actual first fill-up. You should fix your software or at least explain how to add a new car and how to handle the first fill-up. It is not intuitive or straight forward at all.
I never had any bother adding my car or the first fill up. Then on the second fill up it gave me a mpg reading. How do you know how much fuel was in the car when you picked it up? Just because the dealer said there was a full tank, can you be sure?