(a) In lieu of an excise tax on liquefied petroleum gas used to propel motor vehicles over the highways of this state, there is hereby levied an annual flat fee on the following classes of vehicles which require a motor vehicle license using liquefied petroleum gas as fuel. For all other purposes other than the excise tax, the term "liquefied petroleum gas" shall be included with the term gasoline as defined in Section 40-17-322:
Class 1. Passenger automobiles, vans, and trucks and pickups under one ton.....................$75.00
Class 2. Recreational vehicles and vans and trucks one ton or over but with a rear axle carrying capacity of less than 14,000 pounds.......$85.00
Class 3. Bobtail trucks and equivalent vehicles to be defined as any other vehicle having the capacity of carrying a loaded rear axle weight of 14,000 pounds or more.......$150.00
Class 4. Tractor/trailer units..............................................................$175.00
(b) In the event an additional excise tax is implemented for alternative fuel vehicles, other than those powered by compressed natural gas or liquefied natural gas, such excise tax shall also be applicable to liquefied petroleum gas used to propel motor vehicles over the highways of this state in lieu of the annual fee provided for in this section.