Here is an answer I recieved from Bridgestone a year or so ago, posted July 2005.
Cruzer is right on
I got my answer from Bridgestone. Sounds like everyone is on the right track.
Here is my question and their answer.
With air pressure and tire temp sensors installed in aluminum wheels, what kind of temperatures should be observed while driving in 90 to 100 degree temperatures?
What air temperature reading would be too high? We are using the Smart Tire system.
Am-Pac Tire Dist., a BFS distributor in California.
Thank you for the opportunity to be of assistance.
The answer to your question depends on a number of factors, and quite frankly, there is not a hard and fast rule.
There are basically (3) types of tire temperature measurement
Probes inserted into the tire
Contained air temperature (which your system uses)
Tread (surface) temperature
A probe inserted into the tire – into either the belt edge or the bead area – the hottest points of the tire – is the most accurate method; however, it can only be performed under controlled conditions.
The contained air temperature method is the next most accurate, however, it is affected by the mounting system of the sensor – if the sensor is attached to the wheel, it will pick up heat from the wheel (which is picking up heat from the brake drum); and if it is attached to the tire interior, it will pick up heat from the casing.
Tread (surface) temperature is the least accurate, since measurement is normally performed by a hand held unit, thus hampering repeatability, plus the question of where do you measure? The ribs will be cooler than the grooves, and the center will be cooler than the shoulders, etc.
So, while all this has so far done little to answer your question, hopefully it has shed some light as to why I am being a bit reserved in my answer.
Now, what can I say to try to address your question?
While this is not set in stone: A very general rule of thumb is that a properly inflated/loaded tire, when up to operating temperature – one hour+ operation - will typically run about 60 degrees F. hotter than the ambient temperature. Anything above 200 degrees F. could lead to tire degradation and you need to investigate for a problem.
I hope this has answered your question to your satisfaction; if not, or if you have any other questions, please feel free to contact me.