I just wanted to help out with the stock to aftermarket tire ratio on the 235/35/19 fronts & 325/30/19 rears combination. I came with some slightly different #'s on the front sizes. I'm not trying to beat someone in calculations, just helping out with the correct numbers.
I normally look at the diameter of the tires, although the circumference will give you an equal difference.
I find this one useful for a quick overlook and this tire calculator for more details (tick the box 'compare .....'). I'm not too familiar yet with the acceptance limit of Porsche, so someone else would have to fill in on that part.
I guess the best is to have someone else review their own tire setup with similar size and offset of rims. Like that you'd be completely sure about handling/rubbing/differential issues.

FRONT:
235/35-19: circumference 80.04 in
245/30-19: circumference 77.87 in (-2.7%)
255/30-19: circumference 78.61 in (-1.8%)
REAR:
305/30-19: circumference 82.32 in
325/30-19: circumference 83.81 in (+1.8%)

stock rear/front: 1.0285
325 rear/245 front: 1.0763 (+4.6%)
325 rear/255 front: 1.0661 (+3.7%)

So a GT2 setup: 325/30 rear with a 235/35 front would be the closest with stock: 1.0471 (+ 1.8%)
A 325/30 with 255/35 would give a close ratio compared to stock: (1.0249) or with a smaller front tire 245/35 a ratio of (1.0359)

 

Great info. Thanks for doing the calculations. But the question remains as to what exactly are the tolerances of the differential when running front/rear ratios different from stock??

 

“...Increase rear tire width reduces oversteer/ increases understeer, something that you may not want in our 4WD Turbo. If anything tuners tend to increase the front width and keep rear the same (Techart IIRC). I recall reading that Porsche cup/race cars all tend to have relatively bigger front width too.[/quote]”

I’m with Can here...go wider up front for less understeer. With wider rears your front end will be plowing through turns instead of slicing. You will feel this effect in just screwing around in town i.e. u-turns with acceleration. Your front end will want to go straight.

 

Maybe a 245/35 or a 255/35 tire up front could be a good option, as the stock ratio is approached with a difference of less than 1%.

 

I checked out the sidewall of my stock tires after a great track day at Mosport and must say that I'd be nervous with 20s on the track when you see how little rubber there is on the sidewall with 20s!!! I am interested in seeing how "aggressive" you can go with rubber on 19s but I must say that the handling with the stock wheels and tires was more than good enough for me. I doubt I'll go to the track enough to "need" better but who knows...

 

Check out this close-up at speed...19’s with stock 305’s, rear offsets of 44, CMS RS171’s 11.5in rims. After mounting the GT2/GT3RS rims that have an offset of 51 and are 12 inches wide, we feel you could try 315’s (Hoosiers), 325’s would be questionable.

Twenties are terrible for track use...NOTE the sidewall flex on the bottom right rear vs the the top...I think if I had twenties, the rims might be scrapping....

 

VD, those are some incredible shots! Be careful with 325s on these cars, several guys have had lots of differential trouble, sharky knows the car.
Personally, I would stick with the stock N cup sizes, more than enough street or track, the factory already figured it out for you. I ran my fastest times at the glen with that package with a lot of camber (-1.9 fr and -2.3 rear), no issues whatsoever in the turns, straights or esses.

 

Awesome pictures. Looks like the rubber was almost pulled off the tire.

 

Based on the above images, I ended mounting Michelins Pilot Sport CUPs N0 305/30s and 235/35 on the GT2/GT3RS rims.

 

Hey Bonehead, not the greatest picture but this it what it looks like with 12.0x19 inch rear rims (51 offset) and MPSC’s 305/30’s mounted. The fronts are 8.5x19 with 235/35’s. Champion Springs, OEM GT2/GT3RS rims.



This is 11.5x19 offset of 44 CMS RS171’s and Pirelli P Zeros

 

gt2 is rwd 997 turbo is awd 325 won't work on awd

 

VD, what lip do you have on your car? Any scraping issues?

 

Champion Motorsports Werks1 Carbon Fiber front splitter and Champion 997 TT Sport Springs, which drops the car and stiffens it up, not sure how low, but its pretty low.

Scraping issues...you bet. I have to be very careful and know where I’m going and what the location entry and exits are like. I have yet to rip it off, hit the front of it, or curb it...I’m pretty careful. I’ve had lowered cars before so I have learned a few hard lessons already. As for road use its not bad, just pulling in and out of driveways and business lots/gas stations are always areas of concern. You’ll be safe as long as you remember to angle in and out. It also has aluminum skid plates on the bottom which see and feel a lot of use.

I have been able to navigate through the city (SF) several times, ending up in odd places with some steep *** hills that have very flat bottoms/intersections...its still there and no visible damage...yet!

 

Wow. Just saw this reply. That's a fantastic looking turbo. Going off on a tangent in my own thread, how's the quality of you Werks spoiler? Any fitment problems? Does the weave line up?

BTW, still in search of wheels. OEM GT3RS's are still in the running if I can find a good price. And given the confusion/uncertainty with +sizing, I would probably just go with stock sizes.

 

Quality: “Top Shelf”

Fit: seems perfect, although they installed (Champion Motorsports)

Weave: not sure how they did it but its one continuous piece, right and left half’s and the weave/seam lines up perfectly in the center where they meet in a reverse “V”.