the colder the engine runs the less efficient it is.
Fraser
I had been running a little warm in my 93 during the summer, so when the water pump went out, I figured I would also drop the thermostat to 180F. Well, it turns out that the old water pump wasn't moving much water, because one new water pump + one 180 deg thermostat = too cool. The car would barely run in the "NORMAL" band after that unless it was hot outside.