cold all the time?? a bad thermostat, staying open all the time. when you first start the car the engine needs to warm up fast so the thermostat will stay closed so coolant will only be circulated thru the engine and once the coolant has reached a certain temp it will cause the wax pellet in the thermostat to expand and circulate the coolant to the radiator to expel the heat. but if your thermostat is stuck open coolant is constantly being circulated to the radiator from cold start and is not allowing the engine to heat up properly causing poor gas mileage. to find the thermostat find the lower radiator hose (just trace the hose coming from the bottom of the radiator) to the engine block the hose should connect to a small housing held in by 2-3 bolts that's the thermostat housing remove that and inside is your thermostat by the way drain you coolant before you do that.
or your gauge could be bad.