Orginal Gas Tank or??

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ive done this before. its not very advanced, but it works like a charm:
1. get access to your fuel tank from above.
2. use some sort of measuring stick to see how much (in inches) gas you have.
3. fill tank to capacity, remember amount you put in. recheck amount (in inches) you have in tank now. using basic algebra you will know how much fuel you hold.

EXAMPLE- if you had 2" of gas, then added 20 gallons of gas and now have 6" of gas, you have a 30 gallon tank.