Truck Classification Systems

Trucks can be classified in a number of ways. One of the most commonly recognized truck classification systems is the 13-category classification scheme used by the U.S. Department of Transportation’s Federal Highway Administration (FHWA). This system classifies not only tractor trailers, but all other motor vehicles. To further delineate truck classification systems, truck weight and features are also considered.

Axles, Units, and Trailers

Motor vehicles, including tractor trailers, are often classified based on the number of axles, units, and trailers contained. This classification system is used for all vehicles, ranging from motorcycles to multi-trailer tractor trucks. Vehicles are mainly classified based on the number of axles, as well as the number of units, including trailers.


This classification system primarily depends on the number of axles a vehicle has. An axle is a central shaft runs through the center of a wheel. A vehicle’s wheels are mounted on axles. Two wheels are mounted onto the ends of one axle. Therefore, a 6-axle truck will have 12 wheels.

Units and Trailers

Along with the number of axles, FHWA vehicle classification includes the number of units a vehicle has. More common vehicles are considered single unit vehicles, such as passenger cars and buses. Tractor trailer trucks are categorized as two-unit vehicles: the tractor, or power unit containing the engine, and the trailer, which is attached for transport of goods. Tractor trailers may then be considered single trailer or multi-trailer, depending on the number of trailers which are attached to the tractor. Truck tractors without a trailer attached are considered single-unit trucks.

FHWA Vehicle Classification

The FHWA 13-category vehicle classification system is as follows:

  1. Motorcycles: 2 axles and 2 or 3 tires
  2. Passenger cars: 2 axles, may carry 1-or-2-axle trailers
  3. Pickups, vans, and panels: other 2-axle, 4-tire single units
  4. Buses: full length vehicles with 2 or 3 axles
  5. Single unit 2-axle trucks: 6 tires
  6. Single unit 3-axle trucks
  7. Single unit 4-or-more-axle trucks
  8. Single trailer 3-or-4 axle trucks
  9. Single trailer 5-axle trucks
  10. Single trailer 6-or-more-axle trucks
  11. Multi-trailer 5-or-less-axle trucks
  12. Multi-trailer 6-axle trucks
  13. Multi-trailer 7-or-more-axle trucks

Truck Weight and Features

Federal and state truck weight laws exist to regulate tractor trailer weight while traveling on interstate and intrastate roadways. Truck weight laws exist to maintain safety for both drivers and the integrity of U.S. highways and bridges. Truck weight laws are based on a mathematical formula called the Federal Bridge Gross Weight Formula.

Truck Features

The features of a tractor trailer may also contribute to truck classification systems. When used for certain purposes, tractor trailers will be constructed with certain features. For example, refrigerated trucks used to transport food and other temperature-sensitive materials typically feature mechanical refrigeration systems. These systems keep goods at a specified temperature throughout the duration of transportation.


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