1994 Ford Ranger Engine runs too cool

I've noticed a gradual cooling of the average temperature on the engine temp gauge over the course of the past 20K miles. Now it barely moves off "C" (cold). Ambient temp has been 35F and below lately. Changed the t-stat yesterday (what a bear for just a simple item) and problem still exists. Hot air for the cabin is present, but not great. We replaced the radiator due to a small leak 2 years ago. Fluid appears dirty every time we dump and refill the rad (once a year). We don't do a serious flush - afraid of what might break loose and really do some damage. Engine and accessories are quiet - nice and smooth. Gas mileage isn't the best - 16mpg(ish). Would like to see the temp gauge (and cabin heat) run "normal". Your thoughts would be greatly appreciated.

Update: 12/30/2007
The guy at the auto parts store says there are two temperature sensors. One feeds the computer and one feeds the gage. Since the engine seems to run fine, he said to replace the sensor for the gage - $11. Guess I'll try that one and see what happens. Maybe the engine temp is really OK, but it's just indicating too cool.