thermostat?

[Jeremiah Curry]

Howdy all,
 
On a 400 mile roadtrip the other day I noticed that my water temperature
guage was not budging from the very bottom.  It was 20 degrees outside, but
I have never had a problem with the temperature before.  I have also been
getting really bad mileage lately 15 mpg in town and 22 on the highway.  So
I suspected the thermostat was bad resulting in a rich mixture and cold
running.  However, I put cardboard in front of the radiator and the guage
still did not budge and the heater blows warm...so I am not sure if that is
the problem.  The previous owner said he replaced it about 135,000 miles and
i am now at 194,000.
 
I bought a new thermostat since it was only $20, but when I went to replace
it I couldn't figure out any way to get the old one out.
 
Does this sound like a thermostat probleme to the collective wisdom of the
list? and if so, is there a trick for replacing them.  Also, my timing belt
is probably due before too long, any benefit to doing one while I do the
other.
 
thanks,
Jeremiah