The M939 Truck is a 5-ton 6×6 U.S. military heavy truck. The basic cargo versions were designed to transport a 10,000 lb (4,500 kg) cargo load over all terrain in all weather. Designed in the late 1970s to replace the M39 and M809 series of trucks, it has been in service ever since. The M939 evolved into its own family of cargo trucks, prime movers, and recovery vehicles, with about 32,000 in all produced. The M939A2 models use a newer and smaller Cummins 6CTA8.3 504 cu in (8.3 L) turbocharged inline 6 cylinder diesel engine developing 240 hp (180 kW) at 2100 rpm and 745 lbf⋅ft (1,010 N⋅m) torque at 1,500 rpm. All models have an Allison 5 speed automatic transmission with a two speed transfer case. The front axle was engaged in low range. The M939A1 and M939A2 series use oversized 14:00 R20 tires and rear tandem "Super Singles". M939A2 series vehicles use a central tire inflation system (CTIS).