it all depends on how sensitive your thermostat is. I have learned that there is a vast difference between thermostats, no two are alike. Some open at 80, or 85 or even 90 (I had a new one that didnt open until after the temp warning light had come on
), and may not fully close again until they reach 60, others may close at 70. I dont think there is any such thing as a 'perfectly performing' thermostat valve. I also dont think that a thermostat can be fully open at 80 degrees and fully closed at temperatures below that, there will be some leakage between those temps and hence a lower temp when coasting downhill or driving in cold air.
As long as your engine reaches 80 degrees (the time it takes will vary due to our all very different thermostats) and doesnt sit too high or too low all the time, then I think you have a good working thermostat. When your cooling fan kicks in to control temp in traffic or hot weather has to do with the thermatic switch, which again probably have variable tolerances across the board.
If your really worried about engine temperature, install an oil temp sender and gauge for the bigger picture...then there will be a new thread on how hot your engines are getting...
