As Red58 stated, you already have the answer. I assume you are measuring your fuel in U.S. gallons rather than in cubic inches. You could use the formula given and solve for cubic inches if you feel a need to know. Should be about 93 cubic inches shy of 19 gallons.

Reminds me of a story about T.A. Edison asking one of his egghead assistants to determine the volume of a light bulb. A week later Edison asked the assistant if he had the volume. The assistant showed Edison pages of equations and indicated he wasn't quite done. Edison filled the bulb, dumped the contents into a graduated beaker and read out the volume.

Baldy