A red Tractor pulling a trailer.

In North America, semi tractors usually have 3 axles, the front, or “steer”, axle having two wheels, and each of the two rear, “drive”, axles having a pair of “dualies” (double wheels) on each side. Thus, the most common configuration of tractor has 10 wheels, however in some cases dual wheels are replaced by tires known as “super singles” (see below), or wide-base singles, to reduce the weight of the tractor. (The weight reduction is about 180 pounds (82 kg) per axle when using super singles). In this case the tractor will only have six wheels. A smaller tractor, having a single drive axle (six wheeler) is often used to pull shorter trailers in tight urban environments, such as downtown areas where a 60-foot (18 m) rig would be too difficult to maneuver. Sleeper cabs (trucks with sleeper units) usually don’t have single axles, but both day cabs and trucks with sleepers are produced with them.

The cargo trailer usually has two “tandem” axles at the rear, each of which has dual wheels, or eight wheels on the trailer. Many trailers are equipped with movable tandems and fifth wheels that can be set to adjust the weight on each axle to stay within legal limits.

Although the sum of the weights of the tractor and the trailer (the gross weight) is normally limited to a certain amount, some roads are marked with a different gross restriction to prevent road damage. Cargoes that exceed allowed weights are usually marked with overweight load and must obtain a permit to use certain roads.

Rules governing the maximum size and weight of vehicles differ from one U.S. state to another. However, since the majority of hauling is done on the interstate system, the vast majority of trucks and trailers made in the U.S. are built to the specifications of the Department of Transportation (DOT), which governs the use of the interstate system. The DOT has established these vehicle limits: 102 inches (2.6 m) wide, 13.5 feet (4.1 m) feet in height, and 80,000 pounds (36,000 kg) gross weight. ² These limits can be exceeded as individual states have the right to issue temporary oversize and/or overweight permits.