I have just purchased a 1980 300sd with 113,000 miles on it. I also have a 84 300sd with 250,000 on it. The 80 runs at 100 on the temp gauge where the 84 runs at 80 on the temp gauge. Outside temp was 73f today so not to hot. I am in SO fl and this is always a concern to me. I wasn doing a steady 50mph for at least 10 miles and the gauge went from estimated 98 to 102. I am not converting so that is what the gauge reads. I read in a thread on this forumn that 100 is prefered. Since for the lst 6 years I have had 80 as my benchmark is this going to cause a problem for this car. I have a 80 degree thermostat in the garage but if I do not have to install it would not bother me. Idling in the shop doing a trans fluid and filter change it registered just over 80 for 15 to 30 minutes. When I took it out on the road is when the temp went up. My wagon and coupe all run just over 80 also. Could I have a defective thermostat or is a 100 degree rating also a thermostat ?