A tandem unit consists of two trailers pulled behind a single tractor. They are generally not allowed on small or congested roads. Trucks are larger, the ultimate limiting factor being the length, width, and weight permitted on the highways. With the introduction of the diesel engine to trucking and the availability of refined diesel fuel, operating efficiency was greatly improved. Transmissions and drive-axle assemblies for trucks are larger, tougher, and far more intricate than those in automobiles.
Automatic transmissions are available for some smaller trucks.
Larger trucks, however, have manual transmissions, often with from four to ten forward speeds. Except for the front steering wheels, which are single wheels, larger trucks and tractors usually utilize dual-wheels, often in tandem, allowing as many as five axles on a tractor semitrailer with 18 tires. On smaller pickup trucks, the brakes are hydraulic, like those used in automobiles. Larger trucks and tractors, however, are equipped with a sophisticated system of air brakes with a greater stopping ability. Special devices are also used to prevent jackknifing while braking.