Tyre Comfort Calculator

[DJMCUK]

Hey... I found a black folder in the glovebox with a thick book in it which gives instructions how to use the car!! Who knew!

I looked at it around 7.30pm last night, found the section on TPMS, that the "Fill info" takes into account ambient temperature, wheel sizes and tyre types, partial or full load. Amazing!

Dropped the pressures back down to 0.2 BAR below sticker values today, drove to warm tyres, checked "Fill info" and all four were at +0.1 when finished. Close enough I think.

My 981 being May 2014 doesn't have TPMS, so it's all new to me!

[Nuclear Nick]

It’s extraordinary how often this question is asked and answered isn’t it Steve?

For those who may not have seen the previous many posts on this:

The tyre pressures should only be adjusted by consulting the ‘Fill info’ page in the TPM section of the Multi-function Display (MFD). After a short drive this will tell you the difference in pressure that is in your tyres compared to the recommended pressure. If the number is negative then you need to add that amount to that tyre or if positive then let that much out. Bear in mind that this fill info is corrected for tyre temperature so will be correct whatever the ambient temperature. This is why it is better to do it this way than by using a hand held pressure gauge. Of course the TPMS needs to have the correct tyre information already entered in the car’s settings.

I've followed this practice since owning my SD but somehow it always seems counter intuitive. For many years with whatever car I happened to own at the time I would add air in the midst of winter and release it at the height of summer so that the pressures were "correct" at the average temperature for that time of year. As digis' chart shows, tyre pressures at zero degrees centigrade are around 10% lower than at 20 degrees. Porsche seem happy with that and, presumably, so are their preferred tyre manufacturers so who am I to argue.

Peter - From what you’ve said here, either you have misunderstood how the TPMS works, or I’ve misunderstood what you’ve said!

The system is ‘temperature corrected’. That means that it will set a lower pressure in cold temperatures to achieve the correct tyre inflation, ie sidewall deflection, so that the tyre performs correctly in cold temperatures. Vice versa it will ‘overinflate’ tyres in hot temperatures at least in part to prevent the tyre overheating due to excessive sidewall and tread flexing. At 20C ambient the system setting will match the recommended pressure and this is the only time a pressure gauge will give the correct reading.

[On-Track]

Peter - From what you’ve said here, either you have misunderstood how the TPMS works, or I’ve misunderstood what you’ve said!

The system is ‘temperature corrected’. That means that it will set a lower pressure in cold temperatures to achieve the correct tyre inflation, ie sidewall deflection, so that the tyre performs correctly in cold temperatures. Vice versa it will ‘overinflate’ tyres in hot temperatures at least in part to prevent the tyre overheating due to excessive sidewall and tread flexing. At 20C ambient the system setting will match the recommended pressure and this is the only time a pressure gauge will give the correct reading.

Thanks for the clarification, I think I better understand it now.