Confusion with TPMS Tyre Pressure Difference and Tyre Pressure Gauge

[Markjp]

I set my tyre pressures using a tyre pressure gauge to 31psi all round using the comfort setting in TPMS and check the TPMS pressures when tyres are cold. I didn’t realise there was also a setting to check Pressure difference until today. What is confusing me is when the tyres were slightly warm and showing 32psi all round the Pressure difference was saying +3psi all round, Indicating I needed to remove 3psi which would reduce the pressures to 29psi.

Can anybody help explain why this might be?

Mark

 

[Motorhead]

Mark,

I’m not sure if this answers your question, but don’t forget that your recommended tyre pressures are for an ambient temperature of 20 degC, so with temperatures hovering around 0 degC the pressures could drop by as much as 4psi.

Jeff

 

[SV_WDC]

The tyre temp fluctuates once the tyres get heat in them. Maybe TPMS is accounting for that? I find switching the reading to bar to be more precise, since it has a decimal point

 

[Greenman986S]

Your tyre pressure gauge and TPMS will probably give different readings for the same tyre

 

[911hillclimber]

Agree with the above.
All these devices are +/- a house brick.
The tyre pressure will change as the weather ambient temp changes, hence before a good run you should check the tyre pressures are to spec for the day.

Best thing about pressure monitoring is a puncture situation.

 

[Wollemi]

TPMS gives two readings. It gives an approximate pressure in either bar or psi which Porsche say do not use to set your pressures.
It also gives Fill Info reading, accessible only whilst stationary, which they say is the most accurate and the one you should use for setting your tyre pressures because it is adjusted for ambient temperature. The pressures given on the door card are for an ambient of 20ºC.
If you want to use your own tyre pressure gauge then unless the ambient happens to be 20º you will need to adjust the tyre pressure to account for the difference in temperature. The then technical director at Michelin told me that a rough calculation is to reduce the pressure by 0.01 bar for every 1º below 20º ambient, and conversely increase by 1º for every 1º above 20º.
So if your tyre is specified to run at 2.4 bar at 20º, then if ambient is 10º, the tyre pressure should be 2.3 bar, at 0º it will should be 2.2 bar.
Fortunately the Porsche TPMS system does the calculations for you, so if you go by the Fill Info readings you should be right.

 

[Markjp]

Thanks for the replies, I understand what the pressure difference means now!