I've got a 94SE with 124K. The temp gauge needle seems to be running a little lower than it historically has (maybe one needles width - it runs about 1/4" below 200F on the gauge)and my mileage doesn't seem to be as high as it should. Is it possible that the thermostat is stuck open and the engine is not reaching optimum temp? Would that cause lower gas mileage?

Is there a way to check this or should I just replace the old thermostat (am I correct that it would be a 195F thermostat) and see if that makes any difference?

your temperature isn't low enough to cause a bad gas mileage condition.. If it was running below 160 then you might have a problem, but certainly not a little bit under 200

If t-stat has never been changed you might as well do it. There'* no way to tell if your stat is stuck open unless you remove it.
Like Custom said you would have to be running low temp stat to affect gas mileage. When was the last time you did a tune up?

If you DO replace the thermostat, you might as well replace the coolant as well if that has not been done in a few years. Have you ever replaced the O2 Sensor? When was the last time you changed the plugs/air filter/gas filter?

Car has 124K on it. Coolant was changed when new water pump was installed at 121K (8/04). Plugs were changed at 109K (11/02). Air filter at 119K (3/04). Fuel filter at 123K (10/04). O2 sensor at 119K (3/04). Techron added at 122K (9/04). PCV at 119K (3/04). The only thing I can thing of is spark plug wires and thermostat (should have had it done when water pump was replaced-I forgot!).

Any other ideas?

Mike