Tires and Wheels for your Porsche

By Dean Lewellen
reprinted from Going Places, official newsletter of the Arizona Region PCA

I have wanted to address this subject for the last two months, but kept opting out because tires and wheels are such a highly personal, often emotional, and, sometimes very controversial topic among Porsche owners as well as among owners of other marques.  Special tire and wheel combinations along with after-market exhaust system setups have become the primary bolt-on products used by owners of later models to personalize their cars, make them unique, and sometimes improve performance in this era of Federal and State operational mandates.  Some serious Porsche drivers have a dedicated tire/wheel set for the street, another for autocrossing, maybe a set for Driver's Education/PCA Club Racing, and, perhaps even another set for concours.  I do not have the knowledge, insight or space allocation to address all these specialty applications, nevertheless, there is a lot of basic and generic information available these days, especially on tires, to aid the Porsche owner in selecting tire/wheel combinations that not only look great but will also match up with the overall performance of the total automobile.  The serious autocrosser who has already sprung for a set of Kumho tires on a set of Fikse wheels will probably not find much new news in this article, but those of you who are looking for replacement or upgrade of your rolling stock, read on.

First, when shopping for a set of tires, remember that the four, approximately 6 inch by 6 inch, squares of rubber on the bottom of your tires is the only part of your Porsche that is in intimate contact with the road surface on which you are traveling.  All the feedback to your suspension system, your steering, braking, and acceleration components, as well as the feedback to your behind on the seat cushion comes from those small patches of rubber on the road.  So, please select the best quality and performance match you can afford for your intended usage, and in any case, never install a tire of lower quality and performance than the original equipment tire that was delivered on your car.

There is a lot of information imbedded on the sidewall of a tire.  Let's review what the code numbers/letters found on an example tire in my garage mean:
"255/40 ZR 17 94Y; DOT FDUY-M12237; Maximum Load 670 kg (1477 lbs) @ 44 psi Maximum Pressure; Treadwear 160; Traction A; Temperature A".
This translates as follows: 255 millimeter tire section width/the sidewall height is 40% (102mm) of the tire section width, the speed rating is Z (149 mph and over), R is for radial construction, 17 is the rim diameter in inches, 94 is the load index which translates to a maximum load of 670 kg or 1477 pounds per tire at the maximum rated tire pressure of 44 psi, which is also spelled out on this tire; the last three digits of the DOT codes (237) indicate the date of manufacture as the 23rd week of 1997; 160 treadwear code is a comparative number which means that this tire should wear 60% longer than a comparable tire rated at 100, or twice as long as a comparable tire rated at 80 (Federal bureaucracy at work here); traction code A simply means that this tire may have better straight line traction performance than a tire rated B or C (lowest) based on straight ahead braking tests (turning and cornering performance is not factored in); temperature code A means this tire will dissipate heat better and run cooler than a tire rated B or C (lowest) when properly inflated and not overloaded.  All this data added together translates toward the old adage that you will get what you pay for.  Expect to pay near $200.00 and up, each, for tires meeting these specifications.

Your owners manual has an important notice regarding tire aging and usage.  It states, "Under no circumstances should tires older than six (6) years be used on your Porsche".  These higher performance, softer compounded tires will age harden over time (particularly here on the face of the sun in Phoenix) and lose their properties, sort of like an "old" racing tire that hardens after numerous heat cycles.  The owners manual also points out that the wear bars between the tread row blocks will appear as solid bars when the minimum service tread depth of 3/32 inch is reached.  When the bars on any two adjacent tread rows appear on a tire as a solid bar it is time to replace the tires on both sides at that axle location.  Tires should always be replaced in pairs, and, fronts and rears should be from the same manufacturer and of compatible quality to minimize unwanted handling traits.  I cannot stress too highly how important proper tire inflation is for the safety, performance, and durability of your tires, Porsche and personal body parts.  The recommended "cold" inflation pressure for your tires is stated in your owners manual and on a decal on the driver's door jamb, as well.

Tire wear patterns will tell you what is going on if you read and inspect them carefully every 2000 miles or whenever you have the wheels off the car for a thorough cleaning and waxing.  Higher wear rates on the inside treads will occur if you run more negative camber than specified front and back (this is common on cars set up for autocrossing and/or the track) or if the front suspension has developed a toe-out condition.  Higher wear rates on the center treads, particularly the rear, is an indicator that the tire is running underinflated.  This sounds backwards, but the lower aspect ratio, wide tires on later Porsches need to be fully inflated to keep the sidewalls stiff enough to maintain flat tread bottoms under loading and high revolutions.  Higher wear rates on the outside treads of the front and/or rear tires is an indicator that the toe-in is too high.  The rear suspension of the early 993s was setup at the factory with the toe-in towards the maximum to provide more oversteer margin which resulted in accelerated rear tire wear.  My car went through three sets of rear tires in 15,000 miles, but, since the rear toe-in was reset towards the minimum, the current set of rear tires has 14,000 miles with about one third of the tread remaining.  A cupping wear pattern on a tire is an indicator of an out of balance or out of round condition of the tire/wheel combination.  This needs to be addressed quickly as tires, suspension components, and drivers will not tolerate this condition very long without rapid wear out.

Tire technology moves and improves at the same alarming rate as personal computers, DVDs, televisions, wireless telephones and other high tech products of our time.  It would sound and be foolish to make any "this is the best tire" recommendations in this fast developing marketplace.  I would say to stick with the products of the manufacturers who have been approved as original equipment suppliers (OEM) to Porsche.  Weissach engineers do an enormous amount of tire testing under all conditions for each Porsche model before selecting the tire providers for the assembly line in Stuttgart.  This, however, should not exclude tire products of non-OEMs that have appeared on the scene after Porsche completed acceptance testing for the factory run because such tires may out perform the earlier qualified OEM products.  Special use tires for racing, autocross and winter driving often are produced by non-OEMs, as well.

Wheels are the most popular way to personalize your Porsche today, and, the product choices available to carry out this transformation are simply mind boggling.  It would be difficult to decide on a "look" from all the candidates available.  When shopping for a set of new wheels, here are a few items/issues an owner should consider.  First, replica wheels of the Fuchs forged wheel, Cup wheels and other factory patterns may not perform well on your Porsche.  This is an area where looks vs. performance vs. cost is not a good trade off.  Next, try before you buy.  Our cars, even of the same model, may require wheels with a particular/special offset (distance from inside hub surface to bead ledge) for brake caliper or suspension component clearance, or a special spoke shape for the same reasons.  Finally, the tire/wheel combination should be checked out as mounted on your car for rubbing on the body, fender lip, and/or suspension parts under the full range of suspension loading, if possible.

Not so many years ago, BBS was the primary manufacturer of after market wheels that were acceptable for usage on our Porsches.  BBS still offers a wide range of world class, high performance wheels, but they have been joined in the after market world of wheels by a large, ever growing number of very high quality producers of wheels for street, for go, or, for show.  I would not try to list every company name here for fear I would miss your favorite wheel maker.  I only mentioned BBS because they were very early in this surge.  Wheels are like art, and the picture created by your wheel choice on your Porsche will be a rolling statement of your personality.