Steven31371

Hey guys, respectfully I went through all the threads but didn’t see an answer to my specific tire size question.

I want to mount 21’ Monoblock HRE’s or similar...maybe a set of AG Luxury monoblocks I saw...specifically 21x9 and 21x11.5 with offsets that will push the big sidewalls of the SportCup 2’s about a half inch past the fenders...I’m not lowering or planning to.

I have a 991.1 Turbo S Cab pictured below along with the tire sizes and wheels I want. It has the GIAC tune on it now and I’m about to do full bolt ons for added power.

I understand that the overall tire diameter and roll rates front to back need to be within 1-2% of EACH OTHER because of the all wheel drive...but can the overall diameter of both tires be an inch to an inch and half taller than stock for a 991.1 Turbo S?

i don’t want to put the PZero 325/25/21 and 255/30/21 sizes....I want to run the Sport Cup 2’s in 325/30/21 and 275/35/21 with enough offset to clear.

But will the larger overall diameter be a problem despite equal diameters and roll rates front to back???

below are screenshots of my car and what I want to run on it...it’s gunna get wrapped in a way that the red will match up theme wise btw lol.

That’s my baby

Those wheels or something similar is what I’m going for

With tires in these sizes

LargePuppy

According to willitfit.com going from 325/25/21 to 325/30/21 will result in an error of -4.46%

Very nice Turbo btw!

Steven31371

from the stock 305/30/20 to 325/30/21 the roll rate increases by about 5.2%...but the VARIANCE front to back stays within 1% when using 275/35/21’s on the front...

so my specific question is...can you increase the roll rate front and back by about 5.2% and not have problems as long as the front and back roll rates are still about equal to each other?


and thank you btw 👊🏼👊🏼👍🏼👍🏼👍🏼👍🏼

polobai

Yes, theoretically you should be fine-your speedo will be off though.

drcollie

Call up the guys at Wheel Enhancement in California. Not only are they an HRE dealer, they do this sort of thing all the time.

Tay101

I think you are going to have more issues with the front 275/35/21. That tire is way bigger than stock. You will probably rub.

You should run 21x9 front on 255/30/21 or 265/30/21 and 21x12 on 325/25/21 running Michelin Pilot Sport 4s. You will not notice any different in performance compared to the Cup 2's on the street and the diameters are much closer to stock.

Steven31371

Ended up going with a 20 inch of the exact red wheel pictured below.

20-9.5-et 46 fronts
20-12.5 et 48 rears

still deciding on which Michelin Sport Cup Two pairings to go with...there are two options.

Pictured below are three tire pairings...1. Is stock 305 rears with 245 fronts....I posted that as a baseline to show stock front to back aspect ratios...there is a 1.5 to 1.6 % variance stock, so we know the system can tolerate some variance in front to back outside diameter or roll rate.

I spent 3 days questioning every local Porsche tuner and Porsche centric wheel place I could reach...and created an escalation ticket with Porsche that will require the local dealer to get an answer from Germany

but so far the consensus is that stock there is a 1.6 % front to back roll rate variance but the system can tolerate up to a 3% variance.

so with the right offsets one could theoretically run 345/30/20 rears and 255/40/20 fronts in SC2’s tires.
..345/30/20 and 265/35/20 should work too.

to balance weight and grip I’m gunna go with 325/30/20 rears and 265/35/20 fronts on 20/12.5 et 48 rear wheels and 20/9.5 et 46 front wheels

Sal635

This is interesting, thanks for sharing. Just to clarify, roll rate variance references a difference in circumference and diameter of the mounted tires between the front and rear of the car? What happens if you exceed the 3% roll rate variance and does it matter if the car is a 2 vs 4wd or MT vs AWD?

Cheers

Steven31371

It only matters on an all wheel drive car...on AWD cars there is a transfer case that connects the front and rear wheels together sorted like a front to back limited slip differential as I understand it...and the computer tells the car when and how much torque to send to each wheel at the right times to keep maximum power hooked to the ground.

if you dramatically change the total outside diameter or roll rate per mile from the stock front to back ratios it can throw the computer off at best, or worse, explode your mechanical transfer case...

this means when buying aftermarket wheels and tires you have to be very careful
about not going too far out of spec on the front to rear tire diameter ratios on AWD cars ....but nobody knew exactly how much ‘out of spec’ was tolerable without risking severe damage to the car or reducing its performance.

But I’m an analyst for a living so first ‘knee jerk’ answers rarely satisfy me lol

A lot of people here were saying that you must keep the front to back aspect ratio within 1%...but when I calculated it the ‘stock’ aspect ratio has a 1.6% variance front to back...so I knew the 1% that many were quoting could not be true.

so I looked deeper

Sal635

Got it. So it's not only the variance between front and rear but also the variance between the new circumference and the original circumference. New wheels/tire combos need to closely match factory circumference and likely with less than 3% variance.

Steven31371

well yes and no...getting the ratio front to back off by more than 3% can cause catastrophic mechanical failures...but let’s say you increase all four wheels by 3% or more but keep close to stock ratio front to back...in this case you probably won’t cause an epic mechanical failure but your speedometer will be off a small amount...if you get dramatically larger or smaller diapers at all four corners you can cause problems with the computer thinking that the wheels are slipping and then it will reduce your power.