26 C.F.R. § 41.4482(b)-1
Example (1). A is the owner of a truck-tractor. On January 1, 1985, A registers the truck-tractor in three states-X, Y, and Z. For purposes of registering the vehicle in State X, A declares the gross operating weight of his truck-tractor to be 60,000 pounds. The declaration of the gross weight of the vehicle at 50,000 pounds places A's truck-tractor in the State X registration category of 55,000 to 62,000 pounds gross weight. Thus, the registered weight of A's vehicle in State X is 62,000 pounds. At the same time as A registers the vehicle in State X, A also proportionally registers the vehicle under the IRP in State Y. A uses the same declared gross weight of 60,000 pounds for purposes of the State Y proportional registration. Registration in State Y at this declared gross weight places A's truck-tractor in the State Y gross weight registration category of 58,000 to 68,000 pounds. Finally, A registers the truck-tractor in State Z. Registration of vehicles in State Z is based on the unladen weight of the vehicle. During the taxable period beginning on July 1, 1985, A's truck-tractor is not registered in any other state. For the taxable period beginning on July 1, 1985, A must declare a taxable gross weight of no less than 60,000 pounds for purposes of the tax imposed by section 4481(a) because that is the highest declared gross weight for state registration or other purposes. Should A declare to any State agency a higher gross operating weight with respect to the truck-tractor during the same taxable period (except for a special temporary permit), A would then be liable for additional tax as determined under paragraph (c)(3) of § 41.4481-1 .
Example (2). Assume the same facts as in example (1), except that on one occasion during the taxable period, A was issued a special 2-day permit to use his truck-tractor in State Y to haul a load which would give A's unit a total gross weight of 80,000 pounds. A may still declare the taxable gross weight of his unit to be no less than 60,000 pounds because special permits to haul heavier loads on a temporary basis are not considered in determining the taxable gross weight of a vehicle.
Example (3). C owns and has registered in his name 2 trucks which are identical in all respects and which are used to carry the same type of load. The first vehicle is registered only in State X at a registered weight of 73,000 pounds based on a declared gross weight of 70,000 pounds. The second vehicle is registered only in State Y at a registered weight of 68,000 pounds based on a declared gross weight of 65,000 pounds. No other declarations of gross weight are made with respect to either vehicle. For purposes of the Federal heavy vehicle use tax, the taxable gross weight of the vehicle registered in State X may be declared at no less than 70,000 pounds and the taxable gross weight of the vehicle registered in State Y may be declared at no less than 65,000 pounds even though the vehicles are identical.
26 C.F.R. §41.4482(b)-1