[guenner]

The BMW TPMS system shows the absolute pressure in the tire adjusted not only to the temperature but also what it would be at sea level because the sensor sits inside the tire with no connection to the outside.

In contrast, your tire pressure gauge shows the tire pressure difference between the pressure inside the tire and the ambient air pressure outside the tire at your current location based on elevation.

Confusing?

Here's an example from 98lee in Pleasanton, CA at the BMW MOA forum:

Atmospheric pressure at sea level is 14.7psi.

Atmospheric pressure at 8000’ is 10.9psi.

14.7 - 10.9 = 3.8psi.

Your differential tire pressure gauge is reading 3.8psi low because it is assuming 14.7psi atmospheric pressure when you only have 10.9.

At sea level and 20°C (68°F) air temperature, the TMPS value and the value on your tire pressure gauge would be the same.

Your owner manual explains how to set your tire pressure correctly using a manual tire pressure gauge. In short, if the TPMS shows for example 1 PSI too low, you measure the current tire pressure and inflate the tire to 1 PSI higher than the reading from your tire pressure gauge.

And all that fuss because the BMW engineers want to show the correct absolute tire pressure INSIDE the tire which really counts.

 

[guenner]

We need the difference between internal pressure and external pressure to maintain the structure and support of the tire.

That is not correct! Changes in the ambient air pressure does not change the air pressure inside the tire. Same PSI inside the tire = same force on the inside of the tire to push the tire wall out. Now if the outside pressure changes by a lot, eventually the tire would deform. Like the tires on the Tesla car that SpaceX put in orbit.

That's why tire manufacturers use some rigid material within the rubbers of the tire to avoid the tire to bulge.

Yes, a temperature change inside the tire changes the tire pressure.

https://www.goodyear.com/en-US/learn/driving-tips/tire-pressure-cold-weather:

"For every 10 degrees of increased temperature, your tires can be expected to increase by 1-2 pounds of pressure."

 

[guenner]

For me, living at 6,500ft (2.000m), the presserure difference to sea level is -3.5 PSI. Any tour on the motorcycle is typically higher up.

Using a manual pressure gauge at my house, my tires would ALWAYS be under inflated by 3.5 PSI.

You low landers don't have that issue setting your tire pressure near sea level.

I invite you to take a trip up Pikes Peak and check your tire pressure with your manual pressure gauge. :shocked:

 

[guenner]

You wrongly assume the mass of air in your tires should be constant at all elevations.

It is! I am not assuming!

Btw. we are talking about VOLUME not MASS. As long as the tire doesn't have a leak and the tire has not bulged, the volume in the tire stays the same - no matter of the ambient pressure. Hence the pressure IN the tire stays the same.

 

[guenner]

The owner's manual for my 2017 F700GS explains excactly how to set up the tire pressure. First time I heard/read about this after so many miles on rubber tires.

If you use a tire pressure at elevations up to 2,000ft the pressure in the tire may be up to 1 PSI too low - not really significant. So maybe that's the reason this is not mentioned in a car/motorcycle manual?

There are lots of discussions like this one on the Internet which explains this much better than I can do.