Im not sure where the needle should be on the temperature gauge, but it looks low to me. Even after driving for half an hour at 55mph+ speeds, the needle is about at 1/4. Actually, heres a perfect picture I found of what it looks like. Mine is just a little bit higher than the one in this picture.
http://www.thecarconnection.com/imag...rument-cluster
Does that still seem suspicious? Well, im glad the replacement parts are cheap.
How exactly would this effect gas mileage though? I mean, I completely understand things like faulty spark plugs or clogged intake filters causing a problem in efficiency, but a thermostat or temperature sensor?