26 CFR §48.4073-1

The tax does not apply to sales of tires of all-rubber construction (whether hollow center or solid) if they have no fabric or metal reinforcement and do not exceed either of these measurements: (a) 20 inches in diameter measured to the outside circumferences, and (b) 1 3/4 inches in cross-section. The exemption provided by section 4073(a) is to be determined solely on the measurements of the tire and not on the purpose for which it is designed or used.