Your car warmed up. You cannot drive the car 55 miles and not have it either warm up or overheat and blow a head gasket. Now it may not be registering on the guage, that is entirely possible. I replace the ECT sensor all the time in these cars because they are either posting a 123 code for it or they are leaking coolant right through the center of it, did one yesterday. Now this may or may not be your problem but I cannot ever recall replacing a temp guage but there is a first time for everything. As luck would have it though you need to remove the thermostat housing to get to the sender so just go ahead and stuff in a new thermostat while you are there. Look on the top corner of the motor over by the P/S pump and you will see where the upper radiator hose goes into the motor, there are two torx head screws right there. Remove them, pull the housing and house up out of the way, remove the T/stat, now with a 19mm wrench remove the sender. It will facilitate it if you pop the plastic connector off of the end of the sender where it houses the wires, this will allow you to slip a box end wrench over it and remove/install the sender. The sender costs about 50.00 (list) and a Calorstat (OE Volvo) t/stat runs about 20.00 (list). Don't cheap out and put aftermarket parts in it, you will not save much and a crappy thermstat will do far more damage than the 5-10 bucks you will save on it. E-mail me if you'd like.

Mark