The 4 Gallon of reserve when he computer tells you you are empty is a common complaint on all the Altimas from what I've read.

And I've never had a car with an accurate MPG calculation. I always do it by hand and the computer's almost always on the Optimistic side. Only 1 tank into the NAH and it was high by ~ 2mpg

I have read that you can make adjustments to how the computer reads vs real world with one of those scangauge II code readers.