ACTUALLY TO BE TECHNICAL, the thermostat in a car has nothing to do with cooling the water. The thermostat is there to keep the engine from getting to cold! Beleive it or not. But if the engine gets to cold it runs less efficient... so THERE, nah nah
The reason a vehicle will overheat from a bad thermostat is if it is stuck closed, EG: not letting any water through at all...