'What can you fill a barrel with to make it weigh less/'

The only tire ratings is the maximum weight that tire can carry at a specified PSI. Normally an E rated truck tire at 80 psi is rated to carry 3042 pounds maximum. What does your door sticker say? Mine says 70 psi front and 65 psi rear (dually). Those tire pressures, in my truck's case, correspond to the air pressure needed for a maximum load. No use putting in more air if it isn't needed.