In the United States, commercial truck classification is determined based on the vehicle's gross vehicle weight rating (GVWR). The classes range from 1-8. It is also done more broadly under the US DOT Federal Highway Administration (FHWA) Vehicle Inventory and Use Survey (VIUS) standards, which groups Class 1, 2 and 3 as "Light Duty", 4, 5 and 6 as "Medium Duty", and 7-8 as "Heavy Duty".
The Class 2 truck gross vehicle weight rating (GVWR) ranges from 6,001 to 10,000 pounds (2,722 to 4,536 kg). Examples of vehicles in this class include the Dodge Ram 1500 and the Ford F-150. Class 2 is subdivided into Class 2a and Class 2b, with class 2a being 6,001 to 8,500 pounds (2,722 to 3,856 kg) pounds, and class 2b being 8,501 to 10,000 pounds (3,856 to 4,536 kg) pounds. Class 2a is commonly referred to as a light duty truck, with class 2b being the lowest heavy-duty class, also called the light heavy-duty class. 
The Class 3 truck gross vehicle weight rating (GVWR) ranges from 10,001 to 14,000 pounds (4,536 to 6,350 kg). Examples of vehicles in this class include the Dodge Ram 3500, Ford F-350, and the GMC Sierra 3500 only dual rear wheel. The Hummer H1 is another example of a single rear wheel Class 3 truck, with a GVWR of 10,300 lbs.
The Class 4 truck gross vehicle weight rating (GVWR) ranges from 14,001 to 16,000 pounds (6,351 to 7,257 kg). Examples of vehicles in this class include select Ford F-450 trucks, Dodge Ram 4500, and the GMC 4500.
The Class 5 truck gross vehicle weight rating (GVWR) ranges from 16,001 to 19,500 pounds (7,258 to 8,845 kg). Examples of trucks in this class include the International TerraStar, GMC 5500. Dodge Ram 5500, and the Ford F-550
The Class 6 truck gross vehicle weight rating (GVWR) ranges from 19,501 to 26,000 pounds (8,846 to 11,793 kg). Examples of trucks in this class include the International Durastar, GMC Topkick C6500. and the Ford F-650
Vehicles in Class 7 and above require a Class B license to operate in the United States. These include GMC C7500  Their GVWR ranges from 26,001 to 33,000 pounds (11,794 to 14,969 kg).
When domestic light-duty trucks were first produced, they were rated by their payload capacity in tons (e.g., ½-, ¾- and 1-ton). This has led to categorizing trucks similarly, even if their payload is different. Therefore, the Ford Ranger, Chevy S-10, and GMC S-15 are called quarter-tons (¼-ton). The Ford F-150, Chevy 10, Chevy/GMC 1500, and Dodge 1500 are half-tons (½-ton). The Ford F-250, Chevy 20, Chevy/GMC 2500, and Dodge 2500 are three-quarter-tons (¾-ton). Chevy/GMC's ¾-ton suspension systems were further divided into light and heavy-duty, differentiated by 5-lug and 6 or 8-lug wheel hubs depending on year, respectively. The Ford F-350, Chevy 30, Chevy/GMC 3500, and Dodge 3500 are one tons (1-ton).
Similar schemes exist for vans and SUVs (e.g., a 1-ton Dodge Van or a ½-ton GMC Suburban), medium duty trucks (e.g. the Ford ton-and-a-half F-450) and some military vehicles, like the ubiquitous deuce-and-a-half.
Throughout the years, the payload capacities for most domestic pickup trucks have increased while the ton titles have stayed the same. The now defunct 'ton' rating is now used to compare standard sizes, rather than actual weight rating.
European Union equivalent truck classification
The classes are a bit different in European Union countries. Class B (car) licences can be used to drive vehicles with Gross Vehicle Weight Rating (GVWR) of not more than 3500 kg (7700 lbs) and a trailer with Gross Trailer Weight Rating (GTWR) not exceeding 750 kg (1650 lbs), while holders of class BE licences can tow trailers with GTWR greater than 750 kg. Such vehicles are also commonly known as light commercial vehicles (LCVs), and include the Ford Transit, Mercedes-Benz Sprinter and Fiat Ducato.
Thus, LCV licence holders can drive any truck that, in the US, is classified as class 1, and class 2 trucks with GVWR not more than 3500 kg. EU licence holders must have a class C1 (light truck) licence to drive trucks that, in the US, are in class 1, 2, 3 or 4, with trailers with GTWR of no more than 750 kg (class C1E licences to tow trailers with GVWR greater than 750 kg). EU class C1 and C1E licence holders can also drive class 5 trucks with GVWR not exceeding 7500 kg (16,500 lbs), while trucks with GVWR of more than 7500 kg GVWR (except class C1E licence holders eligible to drive a combination of a C1 truck and trailer with weighing not more than 12,000 kg [26,400 pounds]) can only be driven by holders of Large Goods Vehicle (LGV) licences. Holders of class C licences can drive trucks that, in the US, are classified in any of classes 1-8, but are restricted to towing trailers with GTWR at or less than 750 kg. LGV licence holders with CE licences can also pull trailers with GTWR greater than 750 kg. Examples of LGVs are the Scania P-series, Volvo FH and DAF 95XF.
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