o2bcdn

I track my 2012 991 C2S (PSAM, PDCC) several times per year and run in the intermediate group. I have run Pirelli P-Zero and Michelin Pilot Super Sport tires with stock 20" rims in stock 245/35-20 and 295/30-20 sizes. Both have worked well at Road America in the dry and rain, allowing for 145MPH+ speeds at the end of the straight with solid traction for breaking and cornering into turn 1.

I am going to buy a set of track wheels and deliberating on 19" vs 20". I plan to outfit them with tires for use in dry conditions only. The key issue I am running into is finding 19" tires with the rolling diameter the same as stock. Most options will result in the overall tire diameter decreasing about 0.8"...I am wondering what effect that will have on the way the car handles on track. I will use my 20" stock wheels and Pilot Sport 4S tires for street driving and wet track days, so really only thinking about dry for the track tires. For example:

Front stock/street = 245/35-20 = 26.8" diameter (PSS or PS4S)
Front track = 255/35-19 = 26.0" diameter (PSC2 or RE-71R)

Rear stock/street = 295/30-20 = 27.0" diameter (PSS or PS4S)
Rear track = 305/30-19 = 26.2" diameter (PSC2 or RE-71R)

Does anyone have experience running 19" track wheels/tires that decrease the rolling diameter by ~0.8"? I would appreciate any feedback about the effects of the rolling diameter on handling, braking (good/bad). 19" wheels/tires combo will run about 2-4 lbs lighter than the same make/model in 20"...the rotating weight reduction should be positive, just not sure about impact of the smaller diameter.

Thanks!

991carreradriver

I have some comments:

First, I have run both 20's and 19's on various tracks. The 19's perform bette IMO.

Second, I would be less concerned with the aspect ratio in favor of approved tires. The RE71's are not approved by Porsche as the rear tires do not have the recommended load capacity. Given the weight of the 911 and the down force created at speed, I would not feel comfortable at 160mph.

Third, I have run SC2's (on my fourth set), They corner and brake extremely well. The only downside is in the wet and their price. My personal preference is that I don't run my 911 in the wet (I leave that to race days in F2000 cars).

Fourth, have a good race shop set your camber and toe for the track, it will do more for you than the aspect ratio.

Good luck.

GT3_Driver

I'm in the process also on picking tires for the track,I use revolutions per mile from Tirerack.
My biggest concern is keeping the ratio between Rev. per mile, front to back, the same as stock.
This is to ensure the computer doesn't get confused.

I haven't looked at load numbers, but will add it to my spread sheet.
I agree it's part of the deciding factor.

CSK 911 C4S

Just a side note the GT-3 Cup cars are on 18"

Wonder if it's because of race rules or driving performance?

mgtrackin

Bringing back an old-ish thread:

991carreradriver - You mention you've run 19" and 20" tires on track in the past. What model and size tires did you run when on 19"?

RonD

I have used both the RE71R and also the Cup2s 255/35/19 and 305/30/19 on the track. The front tire diameter is .2 less than rear tire diameter, same as stock. Both work well for track duty. The RE's cost less and seem to be popular, but I prefer the Cup2s.

CarreraFahrer

I haven't run RE71R's on track but OtrhoJoe prefers them over SC2's. You can read his comments at the end of the thread below. I'm going to try the RE71R's on 19" wheels when I take my car to the track.

https://rennlist.com/forums/991-gt3-...d-journal.html

Getting back to the main topic of this thread the 19's lower the CG of the car, typically weigh less compared to 20", have more tire options and probably most importantly will lower the overall gearing which will help acceleration in any situation. The non GT3 991's have a fairly high overall gearing so a bit lower gearing should be a good thing on track.

There are not a lot of race slick options in 19" and 20" sizes so most race cars run 18's including newer Cup and Club Sport cars. Many of the rules require 18's as well.

hagfish2

drcollie

The general rule of thumb for tracking a car is select the smallest wheel that will clear the brake calipers for several reasons:

1) Lower cost of consumable tires (and usually the wheels themselves)
2) Lowers the car
3) Larger wheels are generally heavier (Less mass in a smaller wheel). Of course this depends on the wheel make and type.

Large diameter wheels are a fashion statement more than anything. You don't need them on the track (yeah your speedo will be off a bit - no biggie). I'd not even object to 18" or 17" wheels for a track set if they will clear the calipers.

Chris C.

I hadn't considered this angle when I was initially shopping for tires, but for posterity the load capacity of the RE71 in 305/30/19 is the same as the OEM tire or MPSC - 102. The speed rating is only a "W" however so keep it under 160 at your local track . Not a challenge with my mere 400 hp, but perhaps for Turbos or GT cars...

CC