I have a 2010 997.2 (Cab 4S). In all the years I've owned it, I basically ignored the TPM button and kept tires pressured at 'full load', which for better or worse kept things simple. However.... in the interests of tire pressure optimization, what information is the TPM trying to tell me when I have 'partial load' checked? 95% of the time I am the only one in the car, so it's clearly a partial load. The door stamp says 37/44 psi front/rear, but that's full load cold. Based on recommendations from the forum, I have lowered these now to 36/40 psi. Today, the TPM button showed +1 psi in the front and +2 psi in the back (before driving). Does that mean that I should be running 35/38 psi based on some type of 'partial load' formula, or current air temp, or is that what it always shows for partial load? I was never clear on this. Of course once I start driving, they heat up and the pressure increases, so the TPM shows larger differentials which still calculate back to the 35/38 combo. It's 80F. Please explain exactly how this button and info should be used.. Thanks!

 

Pressures should always be measured cold (20 D Celsius - 70 F) with a tire pressure gauge, in your garage.
The TPMS system is designed to detect sudden losses in pressure and prevent you from catastrophic accidents.
The TPMS system is not replacing a tire pressure gauge. Just ignore it for this kind of use.

Yves

 

Thanks- but I think we are talking about 2 different things here. The TPMS is not the TPM. TPMS shows the current tire pressure and will light up with a low pressure warning and yes, I agree that measuring your tire pressures cold manually is the best way to go. However, the TPM is a button selected using the same scrolling bar below the OIL level indicator. The TPM allows you to select SUMMER or WINTER tires, then FULL or PARTIAL load, then it shows this differential off of what the actual tire pressure is. That's my question. The differential it is showing suggests I am running +1 psi high in the front and +2 psi high in the back with cold tires right now, assuming a partial load. Since I have 36/40 in the tires, is the TPM trying to tell me to reduce my pressures by 1 psi in the front and 2 psi in the back to best optimize my pressures? And how is that being calculated? I have never used it to guide my tire pressures before, so trying to understand what it's actually designed to do.

 

The TPM (without the S) is just showing its inability to measure the pressures (absolute or differential). If your external gauge says 36 on both front wheels, then the difference shown on the TPM screen is the measure of the lack of precision of the TPMS sensors.
So, what do you want and what do you trust? Your external gauge or a very imprecise electronic system with aging TPMS sensors ?

Just drive the car and ignore that stuff (TPM).

Yves

 

You can set the TPM for plus and minus "ideal" pressure or for absolute pressure. Personally I prefer absolute. As far as an ideal setting if you talk to 5 different people you will get 5 different answers!!!

 

So there is a question as to the accuracy of the sensors vs an external pressure gauge. The sensors cost $170 each and a gauge costs $20. Why would you trust a $20 gauge over a $170 sensor installed inside the tire? I'm not saying you're wrong, but shouldn't we actually test the theory?

 

I have tested mine multiple times. They are the originals still. My car was an early 2010 making the sensors almost 12 years old (or more considering shelf time waiting for installation). They have always read within 1 psi of a very good tire gauge. I've been amazed at how accurate they appear to be over a wide temperature range.

 

My experience is the same as Wade's.
The TPMS is almost always within 1-2 PSI of my tire gauge readings.

If they disagree widely and you verify with two quality gauges- trust the gauges.
Luckily, my 11 year old TPMS sensors and the system itself continue to display accurate information.

 

You guys are lucky with the second generation of TPMS sensors, lasting 10 to 12 years. On my car (2006), the first generation sensors do not go beyond 4-5 years. Granted, they are less expensive and can be obtained for $80 at a Porsche Dealership.

Precision wise: I use two external (and different) gauges and I am happy when they read the same value. TPMS is just in case of tire problems.

Yves

 

Given the known inaccuracies of these systems, I think the OP was asking why the heck Porsche saw fit to include such adjustments. It does seem a little insane to provide such calibration choices when the accuracy of measurement is so poor and subject to the environment...

 

On the 997.2 TPMS is actually accurate.