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