26 CFR § 41.4482(b)-1 - Definition of taxable gross weight.
(a) Actual unloaded weight -
(1) In general. Actual unloaded weight means the empty (or tare) weight of the truck, truck-tractor, or bus, fully equipped for service.
(2) Trucks and truck-tractors. A truck or truck-tractor fully equipped for service includes the body (whether or not designed and adapted primarily for transporting cargo, as for example, concrete mixers); all accessories; all equipment attached to or carried on such truck or truck-tractor for use in connection with the movement of the vehicle by means of its own motor or for use in the maintenance of the vehicle; and a full complement of lubricants, fuel, and water. It does not include the driver, any equipment (not including the body) attached to or carried on the vehicle for use in handling, protecting, or preserving cargo, or any special equipment (such as an air compressor, crane, specialized oilfield machinery, etc.) mounted on the vehicle for use on construction jobs, in oilfield operations, etc.
(3) Buses. A bus fully equipped for service includes the body; all accessories; all equipment attached to or carried on such bus for use in connection with the movement of the vehicle by means of its own motor, for use in the maintenance of the vehicle, or for the accommodation of passengers or others (such as air conditioning equipment and sanitation facilities, etc.); and a full complement of lubricants, fuel, and water. It does not include the driver.
(b) Determination of taxable gross weight -
(1) In general. The taxable gross weight of a highway motor vehicle is the sum of the actual unloaded weight of the vehicle fully equipped for service, the actual unloaded weight of any semitrailers or trailers fully equipped for service customarily used in combination with the vehicle, and the weight of the maximum load customarily carried on the vehicle and on any semitrailers or trailers customarily used in combination with the vehicle. In the case of a highway motor vehicle that is registered in at least one State that requires a declaration of gross weight to be stated as a specific amount for any purpose (including proportional or prorate registration or the payment of any other fees or taxes), the taxable gross weight of such vehicle must be no less than the highest gross weight declaration (or combined gross weight declaration in the case of a tractor-trailer or truck-trailer combination) made by the registrant in any State with respect to such vehicle. If a highway motor vehicle is registered in at least one State that requires vehicles to register on the basis of gross weight and such vehicle is not registered in any State that requires a declaration of gross weight to be stated as a specific amount by the registrant, the taxable gross weight of such vehicle must fall within the highest gross weight category of such State for which such vehicle is registered during the taxable period. Declarations of weight made in order to obtain special temporary travel permits which allow a vehicle to, (i) operate in a State in which the vehicle is not registered or prorated, (ii) operate at more than a State's maximum statutory weight limit, or (iii) operate at more than the weight that the vehicle is registered in a State, shall not be considered in determining the taxable gross weight of a vehicle.
(2) Buses. For purposes of the tax imposed by section 4481(a), the taxable gross weight of a bus shall be the sum of the weights referred to in paragraph (b)(1) of this section except that “the weight of the maximum load customarily carried” on a bus shall be equal to 150 pounds times the number of units of seating capacity provided for passengers and driver.
(c) Examples. The provisions of this section may be illustrated by the following examples: