I would compute your mileage by the more accurate method of filling up and then dividing your total miles driven by the amound of fuel you used since you last filled up.
The gauges on most cars do not move in direct proportion to the amount of fuel consumed. 1/4 tank on the fuel guage might mean 1 gallon at the low end of the guage.
I usually get 150 miles from the full mark before my needle hits the 3/4 mark. That would be 41 MPG and we know that's not true! After that I generally get about 75 miles per 1/4 tank and it gets less and less as I go toward empty.
