If you don’t check out every day, then you are missing out on some great car stuff – not necessarily directed to Porsches, but a “helicopter view” of the automotive industry, racing, technology and general car stuff.  My favorite spot is Nice Price or Crack Pipe, where they pick a car for sale – normally on Craigslist – and describe it, then let’s us vote on whether it looks like a “Nice Price” or “Crack Pipe.”  

This afternoon Kristen Lee published an article on How to Understand What’s Written on Your Tires. That reminded me about the whole topic, so click on the link, read what Kristen has to say, then come back.  We’ll wait…

“Sparky” at Amelia Island in early March with 16″ 928 wheels.
“Sparky” with stock 15″ wheels – one inch taller than the 16’s

The information on how to tell the tire size is very helpful, especially when combined with speed ratings.  One size number that they don’t put on the tire is the overall diameter of the tire.  The article mentions Tire Rack, and I have them bookmarked as I go there often.  Diameter is a handy thing to know.  For example, our 924S – Sparky – had 16″ Porsche 928 wheels on it until after the Amelia Island shows, where I discovered that the inside rear tires were both slightly hitting the inside wheel well on compression.  I switched back to the original 15″ phone dials that had fairly new tires until I can sort out the 16″ dilemma.  (I really like the look of those wheels.)  So just to see what I was getting into, I checked Tire Rack for the overall diameters of the tires – the 15″ were all 215/65 – 15’s, and a check on Tire Rack told me that the diameter was 25.9″ tall.  My 16″ tires were 24.9″ tall, so the car was 1/2″ taller with the 15’s on it.  Not that you would actually notice, but…a half inch is a half inch.

When putting either the 928 wheels or the Design 90 (D90) 16″ wheels on a 924S, we typically stagger front to rear.  I go with 7″ fronts and 8″ rears.  Therefore, the tire width is different front to rear, and when your tires are directional, that means that you cannot rotate the tires.  Most will see that the tire on the front is a 205/55-16, and the rear is a 225/50-16 – and both are the same diameter of 24.9″. So the car will ride somewhat level on the same diameter tires, although the fronts and rears are different widths.

With all that said, the other thing to look for is mentioned at the end of the article – the date of manufacture.  When you go fishing in the used market, you find cars that will have what looks like great tires, but how old are they?  At the Nort Northam Collection, we recently picked up a nice, low-miles 1999 Boxster.  The tires had little wear on them, but a check of the date showed that the rear tires were new in 2010, but the fronts were new in 2003.  Since the car was going to be used for some spirited mountain driving and some auto crossing, replacement was indicated, even though they looked to be okay.  Time is not kind to tires, and having a tire come apart on you on the road isn’t a pleasant experience.

245/35-15 Hoosiers. Treadwear rating? 00.

Tire treadwear rating – a number between “0” and “600” – indicates to us how “sticky” a tire will be against the road.  The lower the number, the better the grip.  However, lower numbers wear out faster, and that is indicated by the treadwear rating when compared to the warranty. An eighty-four month warranty tire will carry a treadwear number somewhere north of 500, meaning that it is made out of plastic, not rubber.  It will last a long time, but possibly not handle like you would want.  On the other hand, a treadwear rating of 200 will give you great grip, but you will be looking for rear tires at about 10-12,000 miles.  (Don’t ask me how I know this.)  DOT race tires carry treadwear ratings of as low as “0” and as high as “40.”  Not great for wear, but really sticky.

So that is our take on tires, selection, and decision-making.  Check the date of manufacture on your tires and don’t get into a situation where you find out that you are on tires that were made before you got out of grammar school.

Questions?  Let us know…

My 1988 924S Special Edition on the trailer to go to its new home, wearing D90 16″ wheels – 7’s on the front, 8’s on the back.  They didn’t hit the inner fenders on the rear, and we will figure out why really soon!  I like the look of the 16″ wheels on a 924S!

Author: Kevin Duffy, Author and Chief Geek

Kevin Duffy is a retired Associate Professor of Criminal Justice at Daytona State College in Florida and a dedicated car guy. He now spends his time with Porsche 924S, 944's and 968's in his backyard shop. He is active in the Porsche Club of America, and he concentrates on 924S and the 924S Special Editions, doing rescues and restorations.

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