When does a thermostat start to open?

I think I have a defective thermostat. Ever since I put it in the car seems to run too cool, and the stock gauge reads lower too. I took it back out and did the boil/candy thermometer method. It is a 180 degree thermostat, but starts to open at 165, and is fully open at 180 or so. Is this correct? I thought it was supposed to start to open at 180, fully open at around 195 or so.

Its a 180 stat not a 195.180 means its open at 180.Id say 165 starting is a little low but better than getting too hot.Lots of guys here would like to have your problem.Most overheat.If you want it hotter get a 195.My 440 has a 180 and runs just about that all the time.What gauge do you have?If its gactory dont trust it as they are known for being off.Rocky

I went to Napa and got a 185 and it opens at 175.
The aftermarket gauge reads about 179-181 which
is fine for me. It will get to 185-186 when I
put my foot in it.

Sounds about right. I run a Mr. Gasket High-flow 180 thermostat, and my car typically runs at about 175, by the guage.

Thanks for the input guys, Since the time of my post, I did some reading and testing. I found that the thermostat RATING (stamped) indicates when it first begins to open, but is not fully open until temps increase above that, which can be 10-20 degrees. (That also explains why a thermostat is there - to provide a MINIMUM temp for the engine.) I then went out and bought two Stant Superstat 180 degree thermostats, since I read various opinions that they were the most accurate. I boiled them. Guess what? BOTH started to open at 180 give or take a couple degrees, and fully opened at about 195 or so. SO, my original IS defective, since it started to open at about 165 degrees, even though it is stamped a 180. By the way, the defective one was a Duralast that I bought at Autozone, so I probably would avoid that brand in the future. Out to the garage I go to install the new one! Bye now!