1. (a) If a carrier operates tractor-trailer combinations he may elect in his return to compute the tax by one of the two methods set forth below:

   1. (1) Compute the tax for each separate tractor and trailer combination operated as a unit, based on the mileage and the unloaded weight of tractor and the maximum gross weight of the trailer as set forth in section 481.4, supra;or
   2. (2) Compute the tax for each trailer based on the mileage of the trailer. In electing to report under this option, the carrier does not report the maximum gross weight of each individual tractor and trailer combination but, instead, reports on the basis of “combination weight”. If operated with a load, the maximum gross combination weight to be reported is the maximum gross weight for each trailer plus the unloaded weight of the heaviest tractor owned by, interchanged with or leased to the carrier during the reporting period for which a permit is required under article 21 of the Tax Law. If operated without any load whatsoever, the unloaded combination weight to be reported is the unloaded weight of each trailer plus the unloaded weight of the heaviest tractor owned by, interchanged with or leased to the carrier during the reporting period for which a permit is required under article 21 of the Tax Law. If the unloaded combination weight is less than 18,001 pounds, no tax is payable on any mileage operated by the trailer when empty. (See § 481.6, infra.) If the heaviest tractor leased to a carrier was not operated in New York State during the reporting period, such tractor may be disregarded for purposes of “combination weight” unless the carrier was required to obtain a permit for such tractor.

2. (b) A carrier electing either of the above methods of reporting must use such method of reporting on all vehicles used during the taxable period. Such election shall be irrevocable with respect to the reporting period for which the return is filed.

   Example:

   If a carrier operates three tractors the unloaded weight of each of which is as follows:

   Tractor A	12,200 pounds
   Tractor B	11,200 pounds
   Tractor C	10,954 pounds

   and three trailers the maximum gross weight of each of which is as follows:

   Trailer X	49,600 pounds
   Trailer Y	44,000 pounds
   Trailer Z	40,000 pounds

   and each combination traveled 100 laden miles on the public highways of this State during the period, the tax may be computed as follows:

   Option A:
   Tractor	Trailer	Maximum G.W.	Laden mileage	Rate	Tax
   A	X	61,800	100	.023	$ 2.30
   A	Y	56,200	100	.021	2.10
   A	Z	52,200	100	.019	1.90
   B	X	60,800	100	.023	2.30
   B	Y	55,200	100	.020	2.00
   B	Z	51,200	100	.018	1.80
   C	X	60,554	100	.023	2.30
   C	Y	54,954	100	.020	2.00
   C	Z	50,954	100	.018	1.80
   					$18.50

   Option B:
   Tractor	Trailer	Maximum G.W.	Laden mileage	Rate	Tax
   A	X	61,800	300	.023	$ 6.90
   A	Y	56,200	300	.021	6.30
   A	Z	52,200	300	.019	5.70
   					$18.90

Tax Law, § 503, subd. 2