Hmmmmm, that may not be the most accurate way to measure your avg mpg's. Without going crazy to be super accurate on your calcuations, you can do the following:
1) Fill the tank all the way up
2) Reset trip odometer
3) If you have a GPS that also tracks distance travelled, zero that one out
4) Drive to your destination, or wherever, until you need to fill up again (assuming same day - i.e. a day trip of over 100 miles)
5) Pull into gas station.
6) Mark down both how many miles your trip odometer says you've travelled, as well as the distance your GPS says you've travelled. If there is a difference, either take the GPS reading, or average the 2 together.
7) Refill Tank
8) Note how many gallons/liters it takes to fill up
9) Divide the number of miles/kms you travelled by how many gallons/liters it took to REFILL your tank
10) Post up your results