The 5‑ton 6x6 truck, officially "Truck, 5‑ton, 4x4”, was a class of heavy-duty six-wheel drive trucks used by the US Armed Forces. The basic cargo version was designed to transport a 5-ton (4,500 kg) load over all roads and cross-country terrain in all weather. Through three evolutionary series (M39, M809, and M939) there have been component improvements, but all trucks were mechanically very similar. They were the standard heavy-duty truck of the US military for 40 years, until replaced by the Medium Tactical Vehicle (MTV) beginning in 1991.