The tire is presumably rigid so it shouldn’t expand like a weather balloon with increasing altitude and the pressure inside won’t change. However your tire gauge has a diaphragm or transducer which works against ambient pressure. It reads higher at higher altitude. However, the TPMS should read absolute pressure. As I live at 5000 ft, this would explain why the TPMS reads 2-3 lbs lower than the gauge when the tires are “cold” and the ambient temperature is about 20C.And if you live at 7000 ft like me?....the inside the tire sensor can’t compensate for the reduced air pressure surrounding the tire. My tires at a sensor reading of 32psi are firmer than if I was in Houston at the same sensor reading.
That’s why I put cold in quotes. Today, my tires are actually cold, as the temp is 7C. The sun is out though, so I’m going for a ride.When tyre, bike and car manufacturers say ‘cold’ @Dougl, they are not referring to ambient temperature, they mean with the tyre not having been driven to any significant extent, or hasn’t been sitting in the sun. A tyre that has been driven (particularly at high speed) will heat up well above normal summer ambient temperatures. 20 deg C just happens to be the standard to which Shrader calibrate their sensors, rather than a target.
The following is an extract from my car manual (also German, but in a different dialect of Germlish to the BMW Motorrad manuals).
View attachment 109720
No, but they do design them with a baseline starting temp, and know fairly accurately how much pressure increase to see when in "normal" use. So if you set them to the temperature corrected "baseline" then they can fairly accurately tell you what the internal pressure will be at any given internal air temperature. If you set the pressure to the 20C "cold" pressure on a hot (25 or 30C) day, the internal pressure will be lower than what the tire manufacturer designed it to be used at.Do tire manufacturers design their tires for use at 20C?
I would not trust the readout on alot of the electic pumps or the readout on a gas station air pump.I have electric pump that I set to 37 or even 37.5 to get tire pressure to show 35-36.
At gas stations is same story, the value indicated at pump is not what bike's pressure monitor shows.
Replace "baseline" with "control" for the math.Temperatures in the US range from over 100F to -50F. How can there be a baseline?