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Width limit for PSS on stock Boss wheels (non LS)

I would like to replace my finished P-Zeros with Michelin PSS. I have seen several size options on the forum, and my local race tire guy suggests 275/35 front and 305/30 rear on the stock rims. Several questions come to mind:
- can the rear 9.5" wheels handle the 305/30s?
- which set of f/r tires will keep the rake as close to stock as possible?
- are there any rubbing issues with larger front tires and the brake duct kit?
- what traction control consequences will arise from a tire size change (if any)?
- what size combination will keep the speedometer most accurate?
I'm sure there are other questions as well; they just haven't occurred to me yet. This car is my daily driver (15,000 miles/yr) and is driven rain or shine. I will also do 8-10 rack days with it per year. I want to retain the stock wheels.
 

Grant 302

basic and well known psychic
On the stock rims, I'd use 255/40 and 275/35...or the 275/40 with 285/40, if you can live with a lot more sidewall play. The PSS sidewalls are pretty soft compared to the Pirellis.
 

PeteInCT

#LS-378 - So many Porsche's, so little time....
Moderator
2,848
14
Connecticut
- can the rear 9.5" wheels handle the 305/30s? - I would check with Michelin (or TireRack) but I do not think so. I am running 35 series Nittos but I'm on a 10 1/2" wheel. I would suspect that you would be in jeopardy of braking the bead on the tire, especially when creating high lateral G's.

- which set of f/r tires will keep the rake as close to stock as possible? - I run 285/35 Nittos up front. No problems.

- are there any rubbing issues with larger front tires and the brake duct kit? - I have none with my 285/35's

- what traction control consequences will arise from a tire size change (if any)? - Shut TC off and you'll have no problems. ;D Seriously, it will cause no issues. I just run with it off all the time, but my car is track-only.

- what size combination will keep the speedometer most accurate? - Once you move away from a 19" 40 series wheel in the rear your speedo is inaccurate. The more you change the overall diameter of the tire the more the speedo will read higher than you are actually driving.

-Pete
 
Just to add on to what Pete said, I use this calculator to help with my tire purchases.
http://www.1010tires.com/tiresizecalculator.asp?err=98&rsn=usr-snt-emty-str
I hate math so it helps make things simple. Tirerack recommends a 10"-11" width rim for 305's.
 

zzyzx

Steve
299
0
I did all the calculations (particularly rake), and here's the sizes of PSS I have on my Boss on the stock wheels:

275/40-19 F
295/35-19 R

These sizes were in consideration of autox (AS/RTR classes), and I didn't want to affect rake and I certainly didn't want a tire shorter than stock that would decrease max. speed in 2nd gear. Our gear ratios are short enough as it is...

I can tell you from first hand experience that this combo works great on the stock suspension and is a drastic improvement in grip levels over the stock Pirellis.

These sizes do not look shoe-horned onto the wheels, but I think a 305 on the R would.

Unfortunately, the R fitment is a BMW spec tire and is priced as such. :(
 
I have a set of 265/35/19 and 295/30/19 sitting in my garage waiting to be installed. I'm looking forward to improved grip compared to the stock Pirelli's.
 

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