volume of a cylinder = pi*(radius^2)*ht
one cubic inch=0.004 329 004 gallon
1 gallon water=about 8 pounds
subtract the volume of the cylinder approximated by your rim from the volume of the cylinder approximated by your tire, convert cubic inches to gallons and multiply by 8 to get pounds of flotation per tire, multiply by 4 to get total flotation.
I weigh about 3400 pounds with only about 1350 pounds of flotation. Guess I shouldn't try