26 CFR §41.4482(a)-1

1. (a)Highway motor vehicle. The term “highway motor vehicle” means any vehicle that is both:
   1. (1)A vehicle propelled by means of its own motor, whether such motor is powered by gasoline, diesel fuel, special motor fuels, electricity, or otherwise, and
   2. (2)A “highway vehicle” as defined in § 48.4061(a)-1(d) of this chapter.
2. (b)Treatment of certain excluded vehicles. Although trailers and semitrailers used in combination with highway trucks or truck-tractors are not vehicles the use of which is subject to the tax imposed by section 4481(a), trailers and semitrailers customarily used in combination with highway trucks or truck-tractors are taken into account in determining the taxable gross weight of the highway motor vehicle under § 41.4482(b)-1, which is the base of the tax.