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

 

Doing the tire measurements, yes the fronts are almost an inch taller in the 18 inch size and about 1/2 inch taller in the rear. The 18 inch ones are taller.The overall circumference is different from front to back normally. I do believe it is needed for the stability management but definitely needed for the speedo.

 

If you use a tire size calculator, they really shouldn't be. Of course snow tires have a deeper tread.
http://www.miata.net/garage/tirecalc.html

 

Maybe this will help explain height differences.

http://www.tirerack.com/tires/tirete...e.jsp?techid=7

 

In fact if, you look at the specs on tirerack and you compare apples to apples (MPSS in both sizes), they're almost the same - https://www.tirerack.com/tires/tires...ot+Super+Sport

 

My 18s with 40 series Bridgestone tires are virtually the same diameter as my 19s with Michelin tires. There is no noticeable difference when they are standing side by side.