The figure below, from the pooled-fund study [1], provides an illustration of the association between vehicle length and vehicle class. The table below lists the class names and is taken from a MAG study [2], which also provides descriptions of the vehicle classes. The vertical red lines in Figure 1 show one potential set of length bins and how those length bins capture different vehicle classes. For example, the “short” bin in this figure will capture all Class 2 vehicles (Passenger Cars), most Class 3 vehicles (Other Two-Axle, Four-Tire Single Unit Vehicles), some Class 5 (Two-Axle, Six-Tire, Single-Unit Trucks) vehicles and a very few Class 6 (Three-Axle Single-Unit Trucks) vehicles.

Class Names and Descriptions from MAG Internal Truck Travel Survey and Truck Model Development Study

Class Description
Class 1 Motorcycle
Class 2 Passenger Cars
Class 3 Other Two-Axle, Four-Tire Single Unit Vehicles
Class 4 Buses
Class 5 Two-Axle, Six-Tire, Single-Unit Trucks
Class 6 Three-Axle Single-Unit Trucks
Class 7 Four or More Axle Single-Unit Trucks
Class 8 Four or Fewer Axle Single-Trailer Trucks
Class 9 Five-Axle Single-Trailer Trucks
Class 10 Six or More Axle Single-Trailer Trucks
Class 11 Five or fewer Axle Multi-Trailer Trucks
Class 12 Six-Axle Multi-Trailer Trucks
Class 13 Seven or More Axle Multi-trailer Trucks
References:
Minnesota Department of Transportation; Research Services. Loop- and Length-Based Vehicle Classification, Federal Highway Administration – Pooled Fund Program. [TPF-5(192)]. Erik Minge, Primary Author. SRF Consulting Group, Inc. Minneapolis, Minnesota. November 2012 (Minge_2012_MNDOT_Length-based_Pooledfund.pdf)
MAG Internal Truck Travel Survey and Truck Model Development Study. Appendix A. (https://www.azmag.gov/Documents/TRANS_2011-02-25_Federal-Highway-Administration-Vehicle-Classes-With-Definitions.pdf)