Tyre Pressures ... +4 p.s.i.

[james2747]

On my 'aged' E Class I have used 30 p.s.i. all round for my tyre pressures for years, as this is the recommended pressures from Mercedes ... well they actually state 29 p.s.i. for the fronts but I bet most garage air supply's are NOT even accurate to 1 p.s.i.
However, I never noticed until recently the +4 p.s.i. bit on the tyre pressure guide that is on inside of the fuel flap (see bottom left hand corner of the photo).
It states: "Warm tyres up to: +4 p.s.i."
Does that mean if tyre is warm I should set pressures to 34 p.s.i. ... or 26 degrees to allow for tyres heating up? And what does 'warm' mean? Is that a sunny summer day with air temps at 20 degrees centigrade and tyres 'cold' ... or tyres warm from running, after a drive in winter maybe, when outside temperature is say 8 degrees?
4 p.s.i. pressure variance ... on 30 p.s.i. tyres is quite a significant variance (over 13.3% to be precise).
I love the full load and high speed pressure recommendations:
I rarely drive with more than 2 up ... and I do try to keep my speed a little under 130 m.p.h. (my 2.7 litre diesel Merc, over 20 yrs old ... oh, I wish!!)
Any suggestions on the + 4 p.s.i. welcomed.

 

[Blobcat]

Test your pressures after a sustained run and you’ll see the pressure is much higher - it’s why you should always test them cold so you’ve a set baseline.
The +4 is if you need to check them at say a motorway service station - set them 4psi higher because when you’re home and check next day they’ll be ~correct

I also tend to keep mine towards the higher end of the recommended pressures - gives better fool economy

 

[Woodleigh]

I also tend to keep mine towards the higher end of the recommended pressures - gives better fool economy

Snap, I always check mine first thing in the morning when they are cold and run them at 2 psi above the recommended which helps when the temperature plummets.

 

[grayb]

We don't push our GT3 until the tyre pressures have risen a bit - the dash display gives a real-time reading. Quite normal for +5psi when nice and warm. Almost impossible to do this time of year in the rain, unfortunately. Sticky CupSport2 tyres don't grip when cold. Sorry! Digression...
Interesting article here https://www.followourtravels.com/caravan-tyre-pressure/ talks about the '4psi rule' for gauging when your caravan/trailer tyres have the correct inflation pressure.

 

[Blobcat]

We don't push our GT3 until the tyre pressures have risen a bit - the dash display gives a real-time reading. Quite normal for +5psi when nice and warm. Almost impossible to do this time of year in the rain, unfortunately. Sticky CupSport2 tyres don't grip when cold. Sorry! Digression...
Interesting article here https://www.followourtravels.com/caravan-tyre-pressure/ talks about the '4psi rule' for gauging when your caravan/trailer tyres have the correct inflation pressure.

Also really important to get the engine fully warm on a 911 before canning it mercilessly

 

[grayb]

Also really important to get the engine fully warm on a 911 before canning it mercilessly

The gearbox on the 997 GT3 won't let you get over-enthusiastic until it's warm - downright hard work until it is. Oil temperature gauge is the one to watch.