IMO, an engine has to be at least 180 to run efficiently. I'd check the specs for your car, and put in the thermostat the factory had in the car. In a pressurized cooling system, operating temps in the 190 to 195 degree range seem to be pretty much the norm. I'm sure there are some specs someplace that show what temp an engine is most efficient at. Greater efficiency will improve both performance and fuel mileage.