For purposes of the tax imposed by section 4061, unless otherwise expressly indicated:
(a) Automobile truck. The term “automobile truck” includes automobile buses, and truck and bus trailers and semitrailers.
(b) Other automobile. The term “other automobile” means all automobiles other than automobile trucks, and includes trailers and semitrailers suitable for use in connection with passenger automobiles, but does not include house trailers.
(c) Tractor. The term “tractor” means any tractor chiefly used for highway transportation in combination with a trailer or semitrailer.