Typically about 19 to 20 gallons.
1) Buy a 5 gallon Jerry Can.
2) Place 1, 2, 3, 4 or 5 gallons in Jerry Can.
3) Run vehicle almost dry of gas and pull over and fill tank with contents of Jerry Can. Note try not to run completely out of gas, it's bad for a lot of stuff.
4) Drive to nearest gas station and fill the tank completely up.
5) Add gallons from Jerry Can to Gallons at the gas station = Capacity of Fuel Tank.
