Why is the spec for 18" tires taller than 19"?

Tire size question and car computer question.....

The spec for my 2009 C2s denotes both 19" and 18" rims and tires. Why is the 18" tire about 1" taller than the 19" when I place them side by side (upright rolling position)? Shouldn't the spec size/aspect make them the same hight (actually diameter)? I discovered this last weekend when I swapped my 19" summer Michelins for 18" winter Michelins - rims are Porsche OEM.

I don't really understand these numbers as I thought I did

front 235/40 18
rear 265/40 18

front 235/35 19
rear 295/30 19

When you swap tire/rim sizes, you must tell the car computer you are on a 2nd set of tires and the size is now 18". I understand telling the computer that there is a new TPMS set but does it need to know size for stability management? Any insights?

Peace
Bruce in Philly

here's what the tire nomenclature means:
The first set of numbers is the width of the tread (in mm)
The second set is the height of the sidewall as a percentage of the width
The third set is the diameter of the wheel (in inches).

So, on a 235/40/18 tire:
235 mm is the width of the tread
40% of 235 is 94 mm, which divided by 25.4 equals 3.7 inches.
So the overall diameter of this tire is 3.7 in times 2 (7.40 in) plus 18 in = 25.4 in.

For the 265/40/18:
Following the same formula, the overall diameter is 26.35 in.

For the 235/35/19 it's 25.48 in
For the 295/35/19 it's 25.97 in

They are all within 5% of each other which is the spec.

Happy Porscheing,
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

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I dont have my handy-dandy XLS file here in f-ing cold alberta (-24F anyone, anyone?).

But i dont eyeball a 1" difference. I see a 0.5" radius difference in teh wheel, offset by a decrease in the aspect ratio. The two should partially cancel. actually, with farmer's math, .05 x (roughly) 250mm = 12.5mm = 1/2" = an exact cancel. What size are the actual tires you are putting side by side? I'll wager one is not as specified.

Now, as to TPMS operation.

Even if they ARE 1" off, as long as all 4 tires are the same (different) size, the computer will have no clue. All it sees is rpm's.

As to sizing and theory, plug those numbers into the spreadsheet I sent you and see..... it does all the work.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com



Edited 1 time(s). Last edit at 12/04/2013 12:23AM by grant. (view changes)

I am traveling..... Won't be back until Saturday.

But there is no doubt, I put two of the tires together and one was very clearly an 1" taller. Actually my friend saw it before I did as it was pretty obvious. I double checked the tires and they were what I ordered and I double checked my order on Tire Rack. Both tires, 18 and 19 are N rated as stated on the tires. I got the correct stuff.

Does the thicker tread add to the height outside of the specific tire spec?

Peace
Bruce in Philly

..about 4-5/32". I think snows are something like 11-12/32"

As long asall 4 are similarly oversized, no worries though. I run + 3% circumference on my Audi on purpose.

Fewer bent rims.

Grant

gee-lenahan-at-gee-mail-dot-com