Calculating life time average

I was sitting around watching football and decided to work on my "numbers" I have been writing down my data from the screen and odometer on the gas receipts. I hoped to calculate my life time average. To this I had been adding up my total miles driven and the total amount of gas used. I have also been calculating the tank average to compare to the computer calculation to see if I can figure out why they vary from one another. I did the calculations and boom there it was 57.35 m.p.g. life time average up from 53.84. I have lost a receipt. :cussing: 347 miles unaccounted for. I suspect I once knew the answer to my next question but it is long gone, I do not use math enough any longer. How close is an average of the actual tank miles to the a life time average calculated the way I was orginally calculating it. I would think that it is not exact but close and would get closer and closer to the real number as you continue. Some of you whose math skills are more intact than mine leep in here. BTW the average of the averages for 17 tanks calculation put me at 49.79 today for a life time average. The lost data was in mid October while my numbers were still high with the temperatures.