Saddle tank The fuel tank on a truck or tractor.

Semi-trailer Truck trailer supported at the rear by its own wheels and at the front by a fifth wheel mounted to a tractor or dolly.

Setback Axle Front steering axle moved rearward from the generally accepted standard position. Advantages: Shorter turning radius and more of a vehicle's weight shifted to front axle.

Shipping Weight "Dry" weight of a truck including all standard equipment, but excluding fuel and coolant.

Single-Source Leasing Service in which companies can lease drivers and trucks from the same source, rather than having to procure them from different companies.

Sleeper Sleeping compartment mounted behind a truck cab, sometimes attached to the cab or even designed to be an integral part of it.

Sliding Fifth Wheel Fifth wheel mounted to a mechanism that allows it to be moved back and forth for the purpose of adjusting the distribution of weight on the tractor's axles. Also provides the capability to vary vehicle combination lengths.

Sliding Tandem (Slider) Mechanism that allows a tandem axle suspension to be moved back and forth at the rear of a semi-trailer, for the purpose of adjusting the distribution of weight between the axles and fifth wheel.

Space Cushion An area around a vehicle which would allow a driver time to avoid an accident.

Speedability Top speed a vehicle can attain as determined by engine power, engine governed speed, gross weight, driveline efficiency, air resistance, grade and load..

Spread Axle (Spread Tandem) Tandem axle assembly spaced further apart than the standard spacing of 54 inches. The U.S. federal bridge formula favors trailer axles with an eight or nine foot spread by allowing higher weight than on tandems with standard spacing.

Spotter A terminal yard driver who parks vehicles brought in by regular drivers. It I is also a supervisor who watches the activities of the driver on the road.

Synchronized Transmission Transmission with built-in mechanisms to automatically "equalize" the speed of its gears to allow smooth shifting without the need to double clutch.