Why is it a bad idea to leave the 190 stat (or 195 whatever it is) and just put a lower temp fan switch? Wouldn't this keep the car from overheating? I put in a 180 stat and a 200* fan switch and my gas mileage has dropped 100kms per tank! (i'm not sure if it's due to that) I will be swapping in the 190* stat to see if it makes any improvement. From what I've read, its ok to put in a 180* stat and leave stock temp fan switch. Whats the best thing to do?

 

They do 2 completely different things. The thermostat (temp setting) determines when the thermostat opens to allow the heated coolant circulating through the engine to pass back to the radiator for cool down.

The fan switch turns the fan on at whatever temperature it is set at.

Both need the other for proper cooling system function. If I had the choice between the two, I would lower the thermostat rating.

You can lower the fan switch all you want. But that won't change what temperature the coolant inside the engine reaces. All that will do is change the temperature of the coolant passing through the radiator (by moving air over the fins sooner, making for a longer time of "cool down"). So yes, you will run cooler at first, when the fresh coolant from the radiator hits the parts, however, that absobs heat and gets contained by the higher stat rating, so in the end, you really aren't achieving much.

Changing the stat setting however allows for complete coolant recirculation at a lower temperature. That will let the engine run cooler by circulating coolant more often (stat opens "sooner" in relation to when the coolant from the radiator hits the various engine components).

Lower both and you are more or less optimizing the situation. If you could lower both, that would be my suggestion ultimatly, I suppose.