I originally posted this in the technical section, where I suspect not many people will see it.
I would love to find something definitive on whether to adjust tyres pressures for different ambient temperature. 20 years ago technical liaison at Michelin told me that cold pressure was (then) measured at 17° C and it would change buy 0.01 bar for each 1°C. eg. If a tyre was 2.2 bar at 17° then at at 37° it would be at 2.4 bar. (0.01 x 20) Similarly if was measured at 7° then that same tyre would be at 2.1 bar. Ideally the tyres should be adjusted to the correct cold pressure in a garage at 17° and then they will be right, if you have to set them in ambient temperature that is different the cold pressure that you use should be adjusted for this. Since then the standard temperature for "cold" tyres has been changed to 20° Some time later another technical guy, this time at Pirelli, said that this was wrong. he said the cold temperature should be at the current ambient, and you should adjust your tyre pressures depending upon the weather. So if my tyres were correct (say 2.2) when inflated in a warm garage, but then parked outside in the cold at 0° I would have to increase the pressures by 0.2 bar to bring them back up. I have read other advice that says the same thing, adjust them for the ambient you will use them in. Now my current car (Porsche GTS 4.0) has TPMS system, which seems pretty accurate. The pressure displayed concur with my gauges and the calibrated gauge at the Porsche Centre. But the Multi-Function display has a function -"Fill Info" to tell you how much to adjust the tyre pressures, and the manual is clear that it is this display that should be observed, not the pressures displayed whilst driving. The manual also states that The displayed pressures take into account the tyre temperature. Currently my tyres are at 2.2 bar, which is the correct cold pressure for the winter tyres fitted. The manual specifies cold pressure as being at 20°C. But it is cold outside - not much above 0° at the moment. The fill info on the TPMS display says that my tyres needed to be reduced by 0.2 bar, which would reduce them to 2.0 bar at the current ambient. It also gives this information on the My Porsche app. I guess that Porsche agree with what the Michelin guy said all those years ago.
For what it is worth,I’m going to try reducing my pressures as in the fill info; I took it out for a drive yesterday and it was quite squirmy under power, traction control lighting up, so perhaps they are overinflated?
