What is the water temp susposed to run on these things?

Since it's been cold (under 30), I see about 180 max and see drops down to 172 when decelerating. I'm wondering if my gas mileage is suffering, because it's not warm enough. I gained 1mpg with my 6.0 diesel with a new thermostat, when it wouldn't get above 180 and stay at 190.

Do I have a defective thermostat? or are these things cooler running?