I have a 1989 Mustang GT Convertible. It is completely stock. I was driving around last week and I noticed that it wasn't warming completely up according to the temperature gauge. I don't know the proper thermostat increments on the dash but according to the lines the car heats up to the third line in the range from 130 to 270. It is in the normal range according to the gauge. I assumed my thermostat was stuck open so I replaced it with a 180 degree one and I still have the same issue. I know running cool should be a great thing but I know there is something wrong. Any suggestions?