From what I understand, the dash guages aren't very accurate. I wouldn't worry about it as long as you know that the t-stat is functioning and is the right temperature stat. If there is something wrong with your cooling system, you'd be overheating, not the opposite. The thermostat is what insures that the vehicle maintains the right temperature. If it's not opening then you'd be overheating, and if it is working right, then you can be pretty sure that you are running at the right temperature. My Ranger 4.0 always reads closer to the cold side of the guage. For a while I had a 180 stat in to see how it affected performance. With it in, the guage would hardly move. Now I have the 198 stat in like it's supposed to. Also, if you were running too cool, you'd be getting lower gas mileage because the ECU richens the mixture until it reaches optimum temperature. If anything it would probably be the sending unit, which only affects the guage.