How do you know what fuel the dealer put in before you got the car? Did they fill it? Did they just stick a load of fuel in and SAY they filled it?
If you wish to assume the tank was full and that the odometer read zero on the new car (maybe in the US, definitely not the case on the UK), then there is nothing to stop you entering two fill ups on the same day.
Personally, I never calculate the mpg until my second fill up. When I get a new car, I fill the tank to the first click off, record the mileage reading, then at the next fill up I do the same, plus record the amount of fuel, and calculate my mpg. That is exactly what Fuelly does.
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2009 Skoda Fabia Elegance 1.4 16V
 
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