This paper presents a continously variable hydromechanical transmission - CVHT. This is three range split power CVHT with an automatic control system. This transmission consists a base for a mathematical model comprising engine, CVHT and vehicle. The mathematical model takes into account hydraulic pressure losses and flow losses in the transmission as well as inertia of the engine and the vehicle. This model is submitted to the computer simulation considering various load conditions and input signals. The results, shown in the form of graphs allow to state that CVHT makes new solution of the problem of the engine operating condition optimization and further development may bring significant profits.