Quick Tire sizing question

[Gator996]

I'm curently using 265/35/18 rears and want to go to a 285 tire. I think 285/30/18 is the Porsche approved sizing.

Am I correct that only difference between 285/30/18 vs. 285/35/18 is in sidewall height?

Wwould a change to 285/35/18 throw off the speedometer readings? By how much?

[Richard Hamilton]

If my math is correct, and I haven't over-simplified it:

265 is the tire width in mm, and 35 is the section as a percentage of the width.

Therefore the height of a 265/35 tire is 265*0.35=92.75mm

For a 285/30 it will be 285*0.3=85.5mm

18 inches is 18*25.4mm=457.2mm

Diameter of 265/35 is 457.2+(2*92.75)=642.7mm

Diameter of 285/30 is 457.2+(2*85.5)=628.2mm

Difference=14.5mm or about 2%

Edited April 6, 2005 by Richard Hamilton

[ar38070]

It wont effect the speedo very much. If you go to www.tirerack.com and look up the tire that you want to buy and check under the specs tab you can see what the manufacturer states as the overall diameter of the tire. For example for a Bridgestone S-03 a 265/35/18 is 25.4" and a 285/30/18 is 24.8". This is about a 2% difference. You have to check each manufacturer as though theoretically a 285/30 is a 285/30 there are slight differences from manufacturer to manufacturer.

The porsche spec for tire size variation from stock is 4%. Some people have reported problems with ABS and PSM at the 4%. 2% seems to be okay.

[LVDell]

I thought the threshold was 3%???

[Dr. Carrera]

Apart from these sizes a critical thing to bear in mind is the Porsche N-rating of the tire you will purchase. Chances are that when you deviate from standard sizes you will not find the tire with the required Porsche N-rating.

[Hroussard]

I almost think that the N- rating system has been put in place so Porsche can retain / maintain an exclusivity and charge us "purists" for the privilege.

HarryR C2 1999

[LVDell]

The price is the same for N rated as it is for non-N rated tires of the same marque and size (give or take a couple bucks). They don't just pick a tire to be N-rated, they have to be tested and approved as tires Porsche would put there as what they deem to be a tire that meets specific requirements of internal construction, tread compound, tread design, etc, to meet the specific performance requirements of these cars. It's not a marketing ploy to make $$$.

[ar38070]

The threshold is 4% per the PCNA website. I do not have the link but it is in one of the tech articles.

The N rating is marketing. Not for $$$ but to make you feel special. Nobody else does it and you cannot tell me that MB, BMW, Ferrari, Lambo, etc. would not be doing it if it made any difference.

Oh my god! I cannot put any R compound tires on my car and take it to the track as there are no N rated R compound tires and therefore they are not up to Porsche perfomance specifications.

[LVDell]

Actually other marque's do in fact do the same thing. BMW has OEM approved tires. As for your R-Compound wisecrack there is no R compound street tire. The N-rating is for street tires. Racing is not covered under your warranty so you are free to do on the track just know that YOU are resoponsible for and damage caused.

Listen, I don't care what you think about N-spec rating. It is like exhaust, oil, suspension, exclusive options, etc.......everybody has their OWN taste. Some choose to buy OEM parts and approved parts. Some could give a **** as to possible ill effect of them and then cry to Porsche when they damage their car. To each his own.

[ar38070]

BMW and other marques may have approved or recommended tires. However they do not go out of their way to tell the tire manufacturers to mark their tires as "BMW approved" as does Porsche.

Where does it state that the N-rating is for street tires only? Or is only because Porsche chooses to apply it only to street tires? Besides R compound DOT approved tires are legal street tires (Kumho victoracers, Toyo RA1, Hoosier S04, Michelin Pilot Sport Cup, Pirelli Corsa, etc.).

If it is your preference to buy only N-rated tires that is your choice. There are many other tires out there that offer as good if not better performance.

[LVDell]

"If it is your preference to buy only N-rated tires that is your choice."

Not my preference, my option. Sure I would like to branch out but there isn't much out there that has a significant price delta that would sway me. Now if you want to buy tires based on price (like the Kumho) then you are just being cheap, which is ironic since they wear faster. But I will tell you that you will end up spending more on the longhaul.

As for this discussion, then we will agree to disagree.

[NickS]

I've been using this site for years.

http://www.miata.net/garage/tirecalc.html

[Richard Hamilton]

I've been using this site for years.

http://www.miata.net/garage/tirecalc.html

<{POST_SNAPBACK}>

Wow! Looks like my calculations were accurate.

[LVDell]

Nicely done Richard!

[Gator996]

Nick

Nice little resource. Thanks for the link!

:beer: