Simple question... I know GM designed our cars to run hotter to help knock down emissions, but did they also design our cars to run hot to save gas? Is a hot engine more efficient? Has anyone noticed a decline in gas mileage after adding a lower temp thermostat and manual fan switches?

 

the hotter an engine runs, the more efficient it is (by efficiency, i mean a ratio between the net work of the engine and the high temperature reservoir, or the temp the engine operates). however, the hotter it runs the worse it is for performance and engine life.