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Discussion Starter #1
I find it interesting that although the OEM rear wheel choices for the 718 vary from 9.5 x 18 to 10.5 x 20, the rear tire size remains unchanged at 265 mm?

Looking at the info supplied on the TireRack web site, it appears that my OEM tire (Eagle F1) in 265/35-20 is best fit on a 9.5" wide wheel.

For competition, Cayman/Boxster drivers routinely stuff a 255/35-18 on the front 8.5" wide wheel. Note that TireRack shows that size tire to best fit a 9" wide wheel. However, speaking from first-hand experience, I can tell you that it fits and works well on an 8.5" wheel.

So, my question is how wide a tire can we fit to the 10.5" wide rear wheel? Specifically, a 10.5" x 19" wheel?

Looking again at the TireRack info for the OEM tire, that data says a 295/35-xx is best fit on a 10.5" wide wheel. The 295mm tire has a 1.2" wider section width then the stock tire so it would roughly extend 0.6" more on each side. Looking at my rear suspension and wheel opening, I don't see how there would be any interference issue at all. In fact, the tire I'd prefer to run is a 305/30-19. However, its section width is 12.3" versus the stock section width of 9.5" on the OEM 265/35-20 tire.

According to TireRack, the 305mm wide tire is best fit on a 11" wide wheel but as with the 255mm tire on the front 8.5" wide wheel, I believe the 305 will fit the 10.5" wheel just fine.

My concern is would the 305mm tire rub the strut or worse yet, the fender? Note that the OEM 265/35-20 has a diameter of 27.3" while the 305-30-19's diameter is 26.2" so there would be 0.55" additional clearance vertically.

Comments???
 

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There is no simple "answer" to your question.

Just as one example let's look at the Michelin Pilot Sport Cup 2 in 20" with a "245" size. You would think that 245=245, but variations between tires and manufacturers can be significant. The tread width of the Cup 2s can vary from 7.6" to 9.6" for a "245" and a "235" Hoosier can have more tread *and* section width than some "245" Cup 2s.

You also need to consider keeping the relationship of the front and rear diameters constant, and what sizes are actually available.

Which tire are you considering?

According to Tire Rack (all Cup 2s on an 8.5" rim):
245/30ZR20 (90Y) XL - Section Width 9.8" - Tread Width 8.8"
245/35ZR20 (91Y) SL N0 (Porsche),911 GT3,Cayman GT4 - Section Width 9.8" - Tread Width 9.6"
245/35ZR20 (91Y) SL K1 Ferrari - Section Width 9.8" - Tread Width 7.6"

Hoosier R7 P235/35ZR19 79W LL (9" rim) - Section Width 10.2" - Tread Width 9.2"
 

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I find it interesting that although the OEM rear wheel choices for the 718 vary from 9.5 x 18 to 10.5 x 20, the rear tire size remains unchanged at 265 mm?

Looking at the info supplied on the TireRack web site, it appears that my OEM tire (Eagle F1) in 265/35-20 is best fit on a 9.5" wide wheel.

For competition, Cayman/Boxster drivers routinely stuff a 255/35-18 on the front 8.5" wide wheel. Note that TireRack shows that size tire to best fit a 9" wide wheel. However, speaking from first-hand experience, I can tell you that it fits and works well on an 8.5" wheel.

So, my question is how wide a tire can we fit to the 10.5" wide rear wheel? Specifically, a 10.5" x 19" wheel?

Looking again at the TireRack info for the OEM tire, that data says a 295/35-xx is best fit on a 10.5" wide wheel. The 295mm tire has a 1.2" wider section width then the stock tire so it would roughly extend 0.6" more on each side. Looking at my rear suspension and wheel opening, I don't see how there would be any interference issue at all. In fact, the tire I'd prefer to run is a 305/30-19. However, its section width is 12.3" versus the stock section width of 9.5" on the OEM 265/35-20 tire.

According to TireRack, the 305mm wide tire is best fit on a 11" wide wheel but as with the 255mm tire on the front 8.5" wide wheel, I believe the 305 will fit the 10.5" wheel just fine.

My concern is would the 305mm tire rub the strut or worse yet, the fender? Note that the OEM 265/35-20 has a diameter of 27.3" while the 305-30-19's diameter is 26.2" so there would be 0.55" additional clearance vertically.

Comments???
Online Wheel and Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit
Change any values there to specifically suit the tires you'll be getting, and take a visual look at everything that will be affected.

You'll lose ~30mm ride height (no idea if you have PASM sport, which is already 20mm lower) in the back alone, which you'd need to balance out with the front.
A 305/35R19 would be a better fit - wider tire with nearly 0 ride height / diameter change.
 

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Discussion Starter #5
? Looks more like ~13mm to me.
Well, per the fitment calculator the 305/30-19 will gain you 13.95 mm drop in ride height over the OEM 265/35-20... but the point of my question is about width not height. I'm thinking we can fit a 305/30-19 rear tire without any clearance issues. Looking at the amount of clearance surrounding my OEM 265/35-20 and realizing the 305/30-19 will be a tad shorted but much wider, I don't have any concerns about rubbing the fender lip. It also appears there is adequate clearance between the sidewall and the strut/spring perch. But I'd really like to hear others opinions on what they think or even better, have fit.
 

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Ok, 13mm is correct (I was reading total diameter difference, but it's the radius that makes the difference in ride height).But, why are you doing a 305/30 instead of 305/35? Moving up to a 305/35 has nearly identical ride height compared to 265/40 (305/30 has a 5% speedo difference too, which is a lot).I'd be shopping 305/35 instead, unless there's a specific reason you're looking at 305/30...
 

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Discussion Starter #7
Ok, 13mm is correct (I was reading total diameter difference, but it's the radius that makes the difference in ride height).But, why are you doing a 305/30 instead of 305/35? Moving up to a 305/35 has nearly identical ride height compared to 265/40 (305/30 has a 5% speedo difference too, which is a lot).I'd be shopping 305/35 instead, unless there's a specific reason you're looking at 305/30...
Actually its ~14mm, but your point is correct! I plan to use this car in SCCA autocross, not DE's... IMO the current tire of choice is the Bridgestone RE71R, which is only available in 305/30-19. I think I'd prefer a 305/35 as that would help maintain the proper ratio of front to rear tire diameter... but that's not an option with this tire. My real dilemma is do i even need a 305 rear tire... I'm only considering it as the 10.5" width is now a SCCA legal alternative to the 10" wheel and a 305 should fit nicely. That said, I'm not sure the car needs a 305... I need to buy wheels soon for next season and I'm torn between 8 & 10 x 18's or the wider but larger 8.5 & 10.5 x 19's. IMO the down side to running 18" is (with the RE71R) the 285/30 is shorter than the front tire so you have to run a 275/35 in the back and that is not much more tire than stock.
 

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The stock factory Pirelli P Zero (PZ4) 265/40ZR19 has a tread width of 9.4" and the 275/35R18 RE-71R has a tread width of 10.0 inches, so you pick up 0.6" (15mm), so you do get a bit more than the sidewall size would indicate due to the shape of the RE-71R.
 

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Discussion Starter #10
The smaller diameter of the 18s will give you better gearing for autocross. 255/35-18 and 275/35-18 is what I would probably go with.
Well, you, "Black718", and many, many others clearly feel that is the way to go... my only hesitation is I honestly believe that is not the optimum set-up... Rather, it’s the best set-up that could be had given the wheels and tires that were available prior to the release of the 718. Now that the 10.5 rear wheel is an option, we (the 718 autocross community) should consider if there are any new tire options resulting because of this additional 1/2" of wheel width.
 

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I have a 981 but stock tire sizes are same. I use Hoosier A7's on 8.5 and 10" rims running 255/35-18 and 295/30-18 with no PASM issues.
 

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and many, many others clearly feel that is the way to go...
The 305 looks like a pretty good fit for the rear. But I don't see a good front to pair with it.

Unless you have an oversteer problem you don't want to go with an even larger width stagger.

And the 19s will keep you a few hundred RPM further out of the powerband, compared to your competitors on 18s.
 
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