A word of caution - those calculations are known to be horribly unreliable. These high readings could be a sign of something wrong with your car aswell and I think it just might be. To get 45mpg is unheard of. When your gas tank gets low, you'll be reading pathetically low mpg. Its a flawed calculation.
The problems are either one of the two:
Firstly, the car cannot exactly measure the amount of gasoline in the tank. it uses a (very imprecise) float-type apparatus to measure the amount of gasoline in the tank (and how much is used).
Secondly, the MPG measurement is based on the estimated amount of gas used (taken from that imprecise float-gauge in the gas tank), the rpm's of the transmission, what gear you are in, etc.