Dan T.

Original tire diameter on my C2 996 was 25.2" F and 24.8" R. Would it be ok to go to 25.6" F and 25.7" R with a new set of tires? The diameter between front and rear would be closer, but it's an increase of 0.4" to 0.9" in total diameter. I have 17" wheels.

Viken

Dan,

Anything more than a 0.5" change is usually considered excessive. However, on the C2, there are no specific issues to deal with. As such, you just have to live with slightly taller final drive and a small change in speedo readout.

Dan T.

Thanks Viken,

I knew you had the answer!

Mike Schuster

Hi Viken,

The stock 996 Turbo Pirelli tires may violate this rule. According to Pirelli's web site, the Pzero Asimmetrico 225/40ZR18-N3 and 295/30ZR18-N3 have diameters 25.2" and 26.0" for a 0.8" overall difference. The revolutions per mile on these two tires are 826 and 801.

If this is true, then by substituting a smaller diameter rear tire one might achieve a small change in gear ratio and effectively a power boost. For example, the 285/30ZR18-N3 has a diameter of 24.9" and a revs/mile of 836. So this tire would give almost a five percent ratio change, or 21HP if ratio and HP are interchangable. Now it is true that the width is smaller and so rear grip would decrease, but with a naturally understeering configuration of the car the net change might be overall an improvement.

Viken

Mike,

Especially on a car like the 996 Turbo, one cannot substitute tire diameters that easily. Both ABS and PSM computers are programmed based on the factory installed diameters and altering them might create a malfunction of one thing or another.

In Dan's case, he doesn't have PSM and/or AWD. Changing either diameter by about 0.5" will not greatly affect the ABS performance.

Mike Schuster

Hi Viken,

I share your concern. And of course I would never try such a foolish thing myself. :-) But maybe there is in fact more leeway in tire diameters than you might assume. Consider this:

The technical/repair manual states that the approved winter tire sizes for the 996 Turbo are 225/40R18 88H and 265/35R18 93H. The Pirelli Winter 210/240 Sport tires in these sizes have diameters 25.1" and 25.2" with corresponding revolutions per mile of 829 and 825, respectively. The manual does not state which particular brand/models are approved (the N designation appears to apply only to summer tires), but these are the only high performance winter tires in these sizes that I could find on the Pirelli web site.

If you know of anyting in writing from Porsche describing ABS/PSM tire diameter restrictions please let me know.

Adrian

Guys,
If you own a C4 or a TT the restriction should be in your owners manuals. The limit is 6% over the original factory delivered configuration.
This is a Bosch programming limitation by the way.
You can work this out in many ways but the easiest I have found is in wheel rotations per mile.
If your 911 be it 964, 993 or 996 was delivered with a set of wheel/tyres providing 850 rotations per mile then you can go up or down by 6% or around 50 rotations per mile. However you have to go the same way. You cannot reduce the fronts by 50 and increase the rears by 50.
You must maintain 6% all round and front to rear.
As a rule of thumb, increase your wheel diameter reduce the profile and vice versa. Tire Rack provide wheel rotations per mile in their spec sheets so you can easily compare tyres and work out the ranges you can use which are quite wide. WINTER tyres is something you need to care about more than summer tyres,
Ciao,
Adrian
911C4