The HMPT-500 hydromechanical transmission was developed under contract to the US Army Tank Automotive Command (TACOM) and after trials with prototype systems was selected for the M2/M3 FVS in 1977. By 1999, well over 11,000 HMPT-500 series transmissions had been built for the home and export markets. This was developed by Lockheed Martin Defense Systems which became General Dynamics Defense Systems early in 1997.

The HMPT-500 automatically determines speed ratio and engine output based on throttle position and vehicle load, and transmits power with infinitely variable ratios through three forward and one reverse speed ranges. Changes between ranges are made with no change in ratio or interruption in power flow. It provides maximum steering torque in all ratios with a large track speed differential and dynamic braking with full engine retarding torque and hydrostatic retarding.

The design incorporates; service brakes for stopping and parking the vehicle ~which are oil-cooled and mechanically actuated, disconnect clutch for starting engine and towing the vehicle and a Power Take-Off (PTO) for operation of engine-driven accessories.

The original HMPT-500 was replaced in production by the upgraded HMPT-500-3 which has an input rating up to 447 kW (600 hp). The HMPT-500-3 was subsequently replaced in production by the digital electronic-controlled HMPT-500-3EC and is installed in the latest M2 Infantry Fighting Vehicles (IFV), M3 Cavalry Fighting Vehicles (CFV) and the Multiple Launch Rocket Systems (MLRS).

The digital electronic-controlled transmissions are readily adaptable for a variety of engine inputs by incorporating simple software changes. Kits to upgrade the HMPT-500S to the HMPT-500-3 or -500-3EC are available. Also 5,000 of the 600 hp HMPT-500-3, -500-3EC transmissions have been produced. It has also been fitted to the prototype of the private venture Vickers Defence Systems VFM Mk 5 battle tank (developed to the prototype stage but no longer marketed) and more recently it has been selected by the United Defense LP, Ground Systems Division, for the M8 Armored Gun System which was developed for the US Army who subsequently cancelled the programme. United Defense is still marketing this for the export market.

Other recent applications include a T-54/T-55/T-62 repower in which the transmission is coupled to a Cummins diesel engine.

The HMPT-500 is also manufactured in the UK for all European-built (MLRSs). David Brown Defence Equipment have manufactured 382 HMPT-500 transmissions of which 287 are for production MLRSs and 95 initial spares. The UK-built transmissions have an input rating up to 447 kW (600 hp). In addition to carrying out overhaul work on European built HMPT-500 transmissions, the company is also now HMPT-500 series automatic transmissions being overhauled and manufactured for European built MLRS at David Brown Defence Equipment facilities in the UK overhauling these for US Army M2/M3 Bradley and MLRSs stationed in Europe. A kit can be used to upgrade early HMPT-500s to this new configuration.

David Brown Defence Equipment have also repaired and overhauled 448 US Army Bradley and MLRS transmissions and 109 European MLRS. They have also remanufactured 88 European HMPT-500 to the HMPT-500-3E standard.

For applications in vehicles in the 36,288 kg, 800 hp class, General Dynamics Defense Systems has developed the HMPT-500-4EC automatic transmission under contract to the US Army Tank Automotive Command. Vehicle trials have been completed in an automotive test rig and production can commence on receipt of orders.