26 CFR § 41.4482(a)-1 - Definition of highway motor vehicle.
(a) Highway motor vehicle. The term “highway motor vehicle” means any vehicle that is both:
(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) A “highway vehicle” as defined in § 48.4061(a)–1(d) of this chapter.
(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.