When I fill mine, it seems like it takes 2 or 3 gallons after the pump does its automatic shutoff. I get just over 500 miles on a tankful or right around 30mpg. The gauge seems to be pretty accurate when it is really full, about 125 miles per quarter or so. I think it's way too early for you to be worried about that. My mileage has gotten better as I have had the chance to really give it some exercise.

Len
'99 E300TD
44,000 miles