First step would be to find out exactly how much your tank holds.
Get a nice sealed fuel can full of 7 gallons of gas and run the car empty.
Fill it with tracked amounts of gas.
For example, run it really low to where you expect to run out some time soon, then dont drive too far away from a gas station. When it runs out you put 7 gallons in it and drive 10 miles to the nearest gas station and fill it up. What ever the numbers are just note them down.
Total in from gas can minus total estimated consumed during the 10 miles to the gas station (maybe 1 gallon) + amount to fill tank = your total tank volume.
Ive done this with every car i've owned just to find out how large the tank is and how much fuel I have left after the gauge reads E.
