26 U.S.C. § 4482

(a) Highway motor vehicle For purposes of this subchapter, the term “highway motor vehicle” means any motor vehicle which is a highway vehicle. (b) Taxable gross weight For purposes of this subchapter, the term “taxable gross weight”, when used with respect to any highway motor vehicle, means the sum of— (1) the actual unloaded weight of— (A) such highway motor vehicle fully equipped for service, and (B) the semitrailers and trailers (fully equipped for service) customarily used in connection with highway motor vehicles of the same type as such highway motor vehicle, and (2) the weight of the maximum load customarily carried on highway motor vehicles of the same type as such highway motor vehicle and on the semitrailers and trailers referred to in paragraph (1)(B). Taxable gross weight shall be determined under regulations prescribed by the Secretary (which regulations may include formulas or other methods for determining the taxable gross weight of vehicles by classes, specifications, or otherwise). (c) Other definitions and special rule For purposes of this subchapter— (1) State The term “State” means a State and the District of Columbia. (2) Year The term “year” means the one-year period beginning on July 1. (3) Use The term “use” means use in the United States on the public highways. (4) Taxable period The term “taxable period” means any year beginning before July 1, 2029 , and the period which begins on July 1, 2029 , and ends at the close of September 30, 2029 . (5) Customary use A semitrailer or trailer shall be treated as customarily used in connection with a highway motor vehicle if such vehicle is equipped to tow such semitrailer or trailer. (d) Special rule for taxable period in which termination date occurs In the case of the taxable period which ends on September 30, 2029 , the amount of the tax imposed by section 4481 with respect to any highway motor vehicle shall be determined by reducing each dollar amount in the table contained in section 4481(a) by 75 percent.