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The owners manual states a difference in tread depth on one axle must not exceed 30%.

What formula is used to calculate the difference?

eg V1-V2/V1+V2/2x100%

LR= 5mm RR=3mm tread depth



2/4=.5x100=50% tread depth difference

Is this correct?
a 2mm difference in effective sidewall height, means an effective difference of 2mm in the radius. Doign the math this results in about a 1% difference is circumference for a 255/40-17 tires (my rear OEM size. 986). A differential should be able to handle that.

I would interpret their requirement as a % difference in the two treads. so smaller / larger must be >= 0.7. e.g.: 3mm/5mm - 0.6 = fail.

I am quite certain i have had a MUCH bigger difference due to wear on track tires. Is it ideal? no.


Grant gee-lenahan-at-vee-eff-email-dot-net

What is the PCNA formula?

Grant's math = 60%

Blackbird's math = 50%

What is the PCNA formula and the correct % difference in tread depth on one axle for the values(5mm-3mm) cited?
Seems prettty simple to me. Here's what my factory reference has to say:

• When replacing a tyre on an axle, make sure that the tread depth of the new tyre does not differ from that of the other tyre by more than 30 %!

There is no mention of radius, circumference, etc. Just the caution the tire tread depths can't differ by more than 30%.

What is almost always the case at least in my experience is one tire goes flat and can't be repaired and must be replaced. So of course a new tire is necessary. The tech knows the tread depth of the new tire and checks the other tire and if the depth of the worn tire is 30% from new the other tire is replaced.

From the Michelin tire site the tread depth for a 255/40 ZR17 tire (which is what my Boxster would take on its rear wheels and which is the end of the car most likely to suffer a flat tire) is 9.5 32nds of an inch. This is 0.296" or 7.54mm.

30% of 0.296" is 0.089" (2.26mm). Subtracting 0.089" from 0.296" leaves 0.207" (5.27mm). If the "old" tire's tread depth is less than 0.207" (5.27mm) then the "old" tire should be replaced.

Grant gee-lenahan-at-vee-eff-email-dot-net
in the european countries that i know, and certainly switzerland where i spent 30 years of my life, vehicle regulations are far more strict than what i know in north america. in switzerland and germany you can't just go out and buy wheels and put them on your car. they must be approve by the respective country's equivalent of DOT.

this is a discussion that has frustrated me on this board and even ppbb in the past. tires on the same axle must have the same performance characteristics so that the car remains balanced. front to back is not as critical as even poor performance is predictable. so you could theoretically put bicycle tires on front and super wide slicks on back and it would be safer to drive than putting slicks on one side and bicycle tires on the other.

in the case of tread depth, imagine it is raining. the tire with more depth will evacuate water much better than the one with less tread. if you have varying tread depth side to side and you need to brake hard, the car will want to rotate (spin) since the forces on one side will be different than the other. that is the reason for needing to have similar tread depths on the same axle. the laws of physics apply here and they cannot be broken.

of course if you have bicycle tires on the front and wide grippy tires in the rear, you car will not stop effectively but it won't necessarily want to spin. of course braking in a turn is a different ball of wax altogether but that's not the issue here.

MY 2000 S, Ocean Blue, Metropol Blue, Savanah Beige. Bought June 2000 - Sold May 2010
Tire failed in Germany. Car rental company had to replace both tires on the axle.

Good question.

Not being in Porsche's confidence I'm not exposed to Porsche's thinking on this subject. All I have to go by is the rule.

While the rule doesn't explicitly mention circumference, which is what I believe I stated in my post above (though perhaps I could have made this clearer), of course the circumference is affected by even a 30% difference in tread depth.

With a 30% difference in tread depth (from a new tire) means the tire's radius is reduced by that approx 0.090". Thus of course the diameter is reduced by 0.180" and the circumference is reduced.

I don't have the Michelin data sheet handy but I seem to recall the tire diameter (new) is 25". This means the circumference is 78.5". With a 30% reduction in tread depth the diameter gets reduced by 0.180" which makes it 24.82". Now the circumference becomes 77.97". 78.5" vs. 79.97" is a 0.0675% change. I do not have any info on what the ABS, traction control, stability systems require in the way of tire diameter consistency. Given these systems are safety critical I'm sure plays a role ensuring in no way differences in tire diameters can have any material effect on their proper function and this consideration probably (probably) accounts for the 30% tread depth call out.

'course, frogster brings up another possible reason for the 30% figure and at least to me appears to have some relevance.

Although I would like to know, I really don't need to know. All I (and the tech) need to know is the 30% guideline and adhere to that. Maybe it is too "tight" but I have no desire to experiment to find out what I can get away with. When the time comes that this 30% tread depth becomes an issue, for instance when replacing a bad tire, I just want the proper tire or if necessary tires fitted to the car and the tires in every way agreeing with what Porsche has to say on this subject so I can resume using the car knowing the car will behave with the new tires the same as it has behaved countless times before.

Grant gee-lenahan-at-vee-eff-email-dot-net
While 1% doesn't seem like much of an error who knows what the threshold is? Safety critical systems like ABS, traction control, stability management; can have much tighter thresholds, tighter operating tolerances, because property/life is at risk if something goes wrong. Even with a 1% difference this is probably still well within the margin of safety Porsche, or the companies that manufacture/supply the safety critical systems, believe is acceptable.
has a 2-3% difference and works fine. Admittedly that is front to back, which would be far less likely to incite a rotation, but for ABS purposes it is similar.

The side to side restriction could be many things, but if it is more stringent than F/R it typically means they are worried about the differential --- and that is consistent with specifying tread depth, not age, pattern or ??? (which i frankly think is more important).

While tread depth might be a little proportional to grip, that is unclear. In the dry less tread may mean MORE grip. Less tread may mean more heat cycling and harder rubber too, which means LESS grip - but both are only possibilities, not always true.

We could go on forever, but i do not like directives without clarification and substantiation, yet in their effort to "dumb down" manuals, this is what we are left with.

For the record i never mix tire frictions if i can help it. But i'm clear about what I'm not mixing :-) It might be old vs new, worn vs not, summer vs AS, blah, blah.....


Grant gee-lenahan-at-vee-eff-email-dot-net
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