Tyre pressures....hmmmm?

Have just had my rear boots replaced (265/35/18 PZeros) which I had regularly checked the pressure of during their life (approx 12 months/10k) in order to maintain the recommended pressure of 44PSI, yet on examination they had clearly worn far quicker in the CENTRE of the tread than at either side. This type of wear pattern is synonymous with the tyres being over-inflated, but they weren't, so what's going on...? Should I be ignoring the handbook and running them at 42 or 40?? Thanks. BTW, for those who might be interested, I got the pair, inc. fitting, balancing and disposal of the old tyres for £324, which was £50 less than I paid (from the same place) for the last pair, and £20 cheaper than blackcircle, plus I only had to travel 1.5 miles from my home to obtain this! [ ]

Neil, Looks like you got some good life out of yours; mine lasted 7.5k on the rears. Mine were worn in the middle more than the side but we're talking 1 - 1.5mm at most difference. Speaking to the tyre guy who's been changing them for 20 years, it's quite normal. Perhaps it's something to do with the offsets we run? I'd be more worried if the inside tread was more worn than the rest of the tyre, suggesting the alignment was out. SNDa

Just replaced my PZero 265x18 rears (with Michelins). They had done 12K and were worn perfectly evenly, and I run them at the recommended 44psi. Strange that yours should wear like that. Have you checked your pressure gauge, or compared the pressures at different filling stations?

Mine are also wearing perfectly evenly (as measured by an OPC technician), yours are likely to be overinflated. I use a digital pump though which is likely to be much more accurate than that found in petrol stations (if that's what you're using)

I use a digital pressure gauge, plus, the gauge on the electric (12v) pump, which is also digital. They never quite agree, varying by approx 2PSI, so perhaps I should get some additional readings. However, I tend to check with the "pressure only" gauge, only using the pump gauge when I'm actually inflating the tyres - the former always reads higher than the latter... I'll go back and see my tyre fitter, and compare his readings to mine. I wonder if his equipment is regularly calibrated?

ah.. sounds like another thing to check then SNDa

My PZero's only lasted 4k[ ] run @ 39 but they were worn very evenly

I'm figuring that the C2's get through rear tyres rather quicker than C4's...therefore I'm assuming that you (and Blade) both drive C2's...?

One set of mine lasted 3600. Mine used to wear in the centre but I put that to always driving at high speed.....

I also have a C2 and the tyres wear in the centre - I have just had them Nitrogen filled and the steering response seems much better!!! The guy who filled them said thou that they normally run them at about 2psi below recommended figures. Any thoughts??? Ian

Nitrogen fill won't make any difference to feel, so if you are correct then it must be down to pressure. The nitrogen will reduce the increase in pressure when hot, mainly because it will be dry and may reduce any gradual loss of pressure, though a good tyre and rim won't allow much of that in any case.

Went up to Jasmine Porschalink on Thursday to have a set of refurbed Sport Design alloys fitted (to replace my current set) and they inflated my rear tyres to 40psi. Obviously curious I asked why and Brian said that once they're warm they're up to about 52psi, which causes them to over inflate and wears the centre of the tyres - exactly what I experienced! So, I shall run my current rears at 40 and see how they perform.

I checked the tyres over the weekend, and was a little puzzled at the low readings I was seeing and the difficulty I was having getting the pressure up. So I compared the pressure guage with every other guage I could find. The result? It was reading a good 7psi lower than the consensus of the others, which were all within 1-2psi of each other. The lesson? Pressure guages don't last forever ... be aware that they might be letting you down*. -------------------------- *sorry...

That sounds a little strange, BB. My tyres (P-Zero) have never risen higher than about 6psi above normal, even during spirited sessions on the track in ambient 25C temps. JohnC 993turbo

Yeah, I was a bit suspect about the acclaimed rise to 52psi, but I just accepted that principle behind the theory seemed reasonable (that there would be an increase in psi, resulting in an over inflation). As suggested elsewhere in the thread, I'll get some comparisons made between my gauges and the tyre fitters.

From experience of race cars I would tend to agree with the recent posts that 6psi is quite a rise but the TPMS sytem in my new C2S shows a rise from 2.7 bar to 3 bar, about 4.5 psi, after a few miles even though I haven't yet pushed at all (only done 500miles).

Hi I had my geometry setup by Chris at CentreGravity who said the Porsche recommended figure of 44 wears out the insides of the rear tyres quicker, and to always inflate them to 36, same as the fronts which gives an even wear. Hope this is useful. Alex

Up to over 20k on my set of Pzeros, still nowhere near the TWIs... no track use mind, and I do drive like a grandma. Wear seems pretty consistent over the tyre surface.

I got 12000 from each of the last two sets of P-Zeros and they wore perfectly evenly across the tread. I always run them at 44. Personally, I think 36 is a bit low for 18's, as that's 20% down on Porsche's recommendation, but each to his own I suppose.

I have been alittle confused by tyre pressures on 911's. I have gained my experience of tyre pressure through doing track days on motorcycles. I track a ZX6R and run pireilli corsa tyres. The recommended tyre pressures for the road (from memory) are 36f & 42r. However for the track pireilli recommend 30.5f & 28.5r. So the fact is the tyres heat up during acceleration cornering and braking which is visible in the form of melting rubber beading at tyre edges and sticking to the underside from fling of the opturating effect. Decreasing the pressure facilitates a greater tyre "footprint" on a motorcycle. Bear with me.....I fitted some tyre warmers to the tyres at cold track temperatures 28.5/30.5 and waited for them to heat to optimum temperature. I then took the pressure and found that they were up by 15psi! This made it clear why the pressures needed reducing for track use, clearly they would be like a drum if run from road pressures and the footprint would reduce under excessive pressure. This is where my confusion comes in with the car pressures- I did a few hours at Bruntingthorpe at 44psi and as the tyre heated up (not excessively) the tyres sounded as tight as a drum which I didn't feel comfortable about, unfortunatley I didn't have a gauge with me to check them. I am of the opinion that if 44 psi is the recommended pressure then if using them under harsh conditions it would be logical to start them at 44psi, warm them up to operating temperature and then reduce them to 44psi. It would be interesting to see what they cool down to and to see if it is feasible to run them at this temperature on the road. I would welcome any feedback on this, is there someone out there who can back up my theory or can warn me off doing something potetially dangerous and tell me I'm just leaking hot air! Pun intended