Ok, so according to the U.S. government, to "fully protect the vehicle from submersion", you need to add
assistant
It looks like you should add about 6 gallons.  Is that enough?
assistant
How much water should I put in the gas tank of my car?
user
I don’t know how to calculate that but I can tell you how to find out.
assistant
Thanks.  I have a small pickup truck.
user
Got it, let’s see... that looks like a 2002 Chevy S-10.
assistant
Yeah, that's it!  So, how much water?
user