dove

Phil, I run at 34 psi all round mainly due to my very stiff set up,oh matron .

Paul
C2 cab

mark lintott

36 all round but on 18" 265/35 and 225/40 with stiff setup if you know what I mean....

Mark

94 C4 TL coupe Taiwan

Christer

205/50 and 255/40 - 36psi all round, cold.

Kismet

Is there a difference between C2s and C4s wrt tyre pressures ?

When I bought my C4 it had 205/255 16" tyres with pressures of 36f and 40r. It under-steered terribly and felt unstable at speed. Dropping the rears to 36 really improved matters. I've now swopped to 205/255 17" with 36 f and r and it is better still.

Regards

JW Hubbers

Which is still a lot better than what happens with my car every (!) time it's been at the OPC: I run 205/50/17 and 255/40/17, and they insist on putting the 16"-spec tyre pressures in them...

Of course I keep asking for the correct pressure and telling them that Porsche recommends something else for 17" than for 16", but so far they haven't managed it once

Cheers,

JW

JoeW

JW, what are the 16" pressure specs? J

Tremaine

Hi all

I am a newbie (to this forum and as a Porsche owner) and am totaly confused with regards to tyres on my 1990 C4.

My tyres are Bridgestone Potenza SO3's:
255/40/ZR/17 94Y - rear inflated to 37.5 psi (2.5 BAR) - COLD
205/50/ZR/17 89Y - front inflated to 34.5 psi (2.3 BAR) - COLD

I live at sea-level and am not sure what compensation (if any) I should make. I also see that my front tyres differ from most other users.

I don't have a clue what all the numbers mean and would like to know the following:

1. What speed rating are my tyres (What is the maximum speed I can do and for how long)
2. What does the 94Y & 89Y mean?
3. Looking at the tyres the rear is wider than the front so i assume that the 255 means width (in what unit of measurement?)
4. What does the 40 mean?
5. What psi do you suggest I inflate the tyres?

My car seems very bouncy (even after I warmed the tyres by driving at 120 KM/H for 10 minutes)

I hope the above questions are allowed in this forum and if not I profusely apologise.

Regards

Steven

Phil Raby

Steve, these pressures are fine - making slight changes from these figures is not going have much effect on the ride quality. What do you mean by 'bouncy'? Do you notice this on straights or through bends? Smooth roads or bumpy ones? Are you running standard springs and dampers?

Cheers

Tremaine

Hi Phil

Thanks, I am such a newby that I don't know what springs and dampers I have (or even what they are) but I would guess that they are standard.

Getting back to the bouncy bit. Say I travel on a straight road at speeds between 160 Km/h - 250 Km/h the car "jumps" all over the road. I assumed alignment but had it checked and it is fine according to the technician.

Should i travel slow (below 40 Km/h) and brake normally to stop at a stop sign the car would not come to a standstill where you would normally expect it. It will invariably stop either left or right from the mark where I anticipated it would stop. Also travelling slow if I let the steering go, the car will sometimes pull left and other times pull right.

I must explain that I moved to Jeffreys Bay only recently and always had the car serviced in Johannesburg but since the move I will need to sort smaller problems out myself as Johannesburg is about 1200 kilometres from Jeffreys Bay.

I have ordered the latest book written by Adrian Streather "Porsche 911 Enthusiast's Companion" and hope that it will give me a bit more insight.

Thanks for your help!

Steve

JW Hubbers

IIRC 2.5 bar front, 3.0 bar rear cold.

On the track I tend to use hot pressures of 2.5 front and 2.5 back on my 17" wheels.

3.0 rear (cold) on 255/40/17 is uncomfortable on the road.

3.0 rear (hot) is downright dangerous on track, at least on my car (which has the H&R 964Turbo springs and yellow Konis).

Cheers,

JW

DavidI

> 255/40/ZR/17 94Y

255 is the width of the tyre in millimetres
40 is the profile, the height of the tyre from the wheel rim to the tread edge expressed as a percentage of the width, so 255x.40=102mm
ZR is the speed rating and is for vehicles capable of in excess of 240km/h. I think I'm right in saying ZR is being phased out and replaced by V (240km/h), W (270km/h) and Y (300km/h). These figures are the maximum, while the ZR just states '240km/h and above'.
17 is the diameter of the tyre in inches, so a 17" tyre will fit a 17" wheel
94Y is the load rating and refers to 670kg, which means absolutely nothing to me.... obviously load rating increases as speed rating increases. I guess you might choose a higher or lower rated tyre even if both are approved at a certain speed - more compliant or stiffer perhaps.

More info at: http://www.tyres-online.co.uk/techinfo/speed.asp

Cheers,

Phil Raby

Sorry to drag up this old thread.

It's been suggested to me that, by running at less than recommended tyre pressures, I could be breaking the law, which could have repercussions in the event of an accident.

Anyone know if this is likely?

Cheers

Wachuko

17" - 36 Front/40 Rear