As used in this chapter the following terms shall have the following meanings:
(1) FUEL ECONOMY. The applicable model year combined highway/city fuel economy with respect to a vehicle as determined on a gasoline gallon equivalent basis as determined by the Administrator of the U.S. Environmental Protection Agency with respect to the vehicle. If such data is not available from the administrator, the fuel economy shall be the total in-use fuel consumed divided by the total miles driven for the most recent fiscal year.
(2) HEAVY DUTY VEHICLE. Any motor vehicle, licensed for use on roadways, having a manufacturer's gross vehicle weight rating greater than 14,000 pounds.
(3) LIGHT DUTY VEHICLE. Any motor vehicle designed primarily for the transportation of persons and with a manufacturer's gross vehicle weight rating of 8,500 pounds or less.
(4) MEDIUM DUTY VEHICLE. Any vehicle having a manufacturer's gross vehicle weight rating of 14,000 pounds or less and which is not a light-duty truck or passenger vehicle.