A hybrid drive system for a vehicle, comprising an internal-combustion engine, a generator, an electric motor, an electric energy accumulator, and a connection device for establishing mechanical connections between the internal-combustion engine, the generator, the electric motor and the vehicle, and a control device. The control device controls a converter device and, as required, the connection device such that the internal-combustion engine is used exclusively for driving the vehicle only if it operable in an operating range in which the operation could also take place only with an electromotive drive, with a specific fuel consumption ESVE lower than ESVL/(G.times.B.times.M), wherein ESVL is the effective specific fuel consumption with which the generator was driven during the charging of the energy stored in the electric energy accumulator, G is the efficiency with which the generator converts the mechanical driving power into electric power, B is the efficiency with which the electric power generated in the generator is stored as energy in the electric energy accumulator and is supplyable to the driving motor, and M is the efficiency with which the electric power in the driving motor is convertible into mechanical energy.