My thoughts would be that your figures are coming from the gauge on the dash, which is nothing more than a vacuum operated (IIRC) guesstimate.
You need to do a real measurement by filling up, resetting your trip odo to zero miles, and driving. Then, top up your tank again, and divide the exact number of gallons you put in by the number of miles traveled.
Anything else, including the gauge or the instrument cluster computer, is just a guess, and not accurate.
There was a guy a few months ago making similar claims about milage, and it turned out he was just going by the gauge, and not really calculating the real number. He had several people freaking out, and a lot of others scratching their heads until I finally asked exactly how he was getting his numbers.
If you are correctly calculating your numbers, then they are extremely good, if not too good to be true.
If you are going by that silly gauge, then I can do one better. I hav seen more than 50mpg on my car for seconds at a time.