Abstract: Motor vehicle hydrostatic transmission having an internal oil expansion chamber including a main housing having at least one opening, this main housing including a sump pump oil reservoir having a predetermined volume of sump pump oil and an input cover sealingly attached to an opening in the main housing, this input cover having an internal oil expansion reservoir which captures or traps a specified volume of air above the sump pump oil level. Thus, when the oil level rises due to elevated operating temperatures, this captured or trapped air volume is decreased resulting in an increase in both the captured or trapped air pressure and the sump pump oil pressure. This causes an increase in the hydrostatic transmission's operating efficiency since the sump pump oil pressure acts as a charge pump to supercharge the low pressure inlet of the closed loop hydraulic system. In addition, the hydrostatic transmission bearing performance will increase since they are designed to operate under pressure.