thermostat since you are replacing the coolant temp sensor. Proper temps need for the ECU for all sorts of things including fuel flow when cold or hot. Poor gas mileage has been due to a faulty theromstat. Stick with the standard OEM replacement that starts to open at 170 and fully open at 194. Check for proper operation of the new on the stove with a pot of water with thermostat suspended in water and a thermometer. The thermo opening specs are in the shop manual here