A cooling system for a heavy road vehicle comprising a hybrid propulsion system, said hybrid propulsion system comprising - a first, mechanical propulsion system comprising a mechanical drive train including an internal combustion engine which provides a traction force to a first traction wheel via a gearbox, - a second, hydraulic propulsion system comprising a hydraulic pump unit for powering a hydraulic motor in order to provide a traction force to a second traction wheel, wherein said cooling system further comprises a pump and a first cooling circuit including a gearbox cooling circuit, a hydraulic propulsion cooling circuit and a first radiator for cooling of a coolant flowing in the first cooling circuit, said radiator being connected in series with and located upstream of the gearbox cooling circuit and the hydraulic propulsion cooling circuit, characterized in that - said hydraulic propulsion cooling circuit and the gearbox cooling circuit are connected in parallel, wherein - a temperature dependent thermostat is positioned downstream said hydraulic cooling circuit, said temperature dependent thermostat being configured to increase the coolant flow through the gearbox cooling circuit relative the coolant flow through the hydraulic propulsion coolant circuit if a measured temperature of the coolant flow downstream said gearbox cooling circuit exceeds a predetermined threshold temperature, and wherein the cooling system further comprises a control unit configured to turn off the hydraulic propulsion system if a measured temperature of the hydraulic propulsion cooling circuit and the gearbox cooling circuit is above a predetermined temperature threshold limit. - a first, mechanical propulsion system comprising a mechanical drive train including an internal combustion engine which provides a traction force to a first traction wheel via a gearbox, - a second, hydraulic propulsion system comprising a hydraulic pump unit for powering a hydraulic motor in order to provide a traction force to a second traction wheel, wherein said cooling system further comprises a pump and a first cooling circuit including a gearbox cooling circuit, a hydraulic propulsion cooling circuit and a first radiator for cooling of a coolant flowing in the first cooling circuit, said radiator being connected in series with and located upstream of the gearbox cooling circuit and the hydraulic propulsion cooling circuit, characterized in that - said hydraulic propulsion cooling circuit and the gearbox cooling circuit are connected in parallel, wherein - a temperature dependent thermostat is positioned downstream said hydraulic cooling circuit, said temperature dependent thermostat being configured to increase the coolant flow through the gearbox cooling circuit relative the coolant flow through the hydraulic propulsion coolant circuit if a measured temperature of the coolant flow downstream said gearbox cooling circuit exceeds a predetermined threshold temperature, and wherein the cooling system further comprises a control unit configured to turn off the hydraulic propulsion system if a measured temperature of the hydraulic propulsion cooling circuit and the gearbox cooling circuit is above a predetermined temperature threshold limit.