NickS

I'm using them for track wheels. 205/50-17 (F) and 255/40-17 (R) are OEM sizes.

LVDell

What are your rim widths? That will dictate what you can put on those rims.

NickS

Sorry... OEM 17" twists... 17x7.5 (F) and 17 x 8.5 or 9 (R)

LVDell

225/45/17 on the front 7.5" (0.4% diamater diff which is perfect)
265/40/17 if it is a 9" rim (1.2% difference..still very good)
255/40/17 if it a 8.5" rim

NickS

harris

could you please why you compare the width with the diameter of the wheel? am I correct?

if yes, could you please tell us the formula?

many thanks

LVDell

What is important in buying tires is that the diameter is within set tolerances. Usually inside of 3% is the gold standard but I think Porsche allows up to 4%.

Formula is as follows:

Tire Diameter in inches= (Width of Tire*Aspect Ratio of Tire*2)/25.4+Rim Size

For Example a 285/30/18 would look like this:

(285*.30*2)/25.4+18=
24.732"

NickS

Or you can just go here.

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

LVDell

No fair, that's cheating

NickS

Haha! I've been using that site for years.

LVDell

Out the window goes my trusty calculator.

NickS

Okay so does anyone think the below is unsafe? Again, OEM 17" twists for the Carrera. Yokohama A032R's for the track.

17x7.5 (F)
17x8.5 (R) I need to double check the wheel width on the rears though.

245/45-17 (F) requires a 7.5" wheel.
275/40-17 (R) requires a 8.5" wheel.

LVDell

Why would you think they would be unsafe? They are within tolerance. You might dial in more oversteer but that's about it.

NickS

I know... it's just me being careful.

I ordered the tires about 30 minutes ago.

arenared

I don't think many manufacters spec 8.5" as an approved rim width for a 275/40R17, but that's not to say it can't be done. It's just going to be a really tight fit. You actually want to run near the wider end of the approved rim widths for performance reasons. Check TireRack for actual diameters and approved rim widths as there is a lot of variability from theoretical.