970 - wheel size tolerances
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Track Day
Joined: Jul 2024
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From: NC
970 - wheel size tolerances
Has anyone CODED non-stock tire sizes via PIWIS (or another system if other coding tools can code such attributes) on a 970? I
f so, can you also name the specific systems it actually affected? Candidates may include systems such as TPMS, PASM, speedo, AWD/diffs, ABS...
observation 1: the stock wheel/tire combos for my 970.1 Turbo, both 19" and 20", measure almost exactly 28" in diameter. Looks from the original catalogs of stock/optioned wheel sets that this was true of all choices.
observation 2: the AWD system did NOT like a set 0f 255/35ZR19 front 305/30/ZR19 rears. when the car hit a hard bump, the AWD system would fail over to RWD. The diameters did not deviate from each other much but they did deviate from 28" stock by about 7%, which I hear ppl say should stay under 5%. Either the tires were striking the wheel well, or, any sort of bounce confused the AWD system, likely via a sensor reading.
observation 3: a set of 265/40 fronts and 305/35 rears, each about 0.5-0.75" smaller than stock, caused no such faults.
f so, can you also name the specific systems it actually affected? Candidates may include systems such as TPMS, PASM, speedo, AWD/diffs, ABS...
observation 1: the stock wheel/tire combos for my 970.1 Turbo, both 19" and 20", measure almost exactly 28" in diameter. Looks from the original catalogs of stock/optioned wheel sets that this was true of all choices.
observation 2: the AWD system did NOT like a set 0f 255/35ZR19 front 305/30/ZR19 rears. when the car hit a hard bump, the AWD system would fail over to RWD. The diameters did not deviate from each other much but they did deviate from 28" stock by about 7%, which I hear ppl say should stay under 5%. Either the tires were striking the wheel well, or, any sort of bounce confused the AWD system, likely via a sensor reading.
observation 3: a set of 265/40 fronts and 305/35 rears, each about 0.5-0.75" smaller than stock, caused no such faults.
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Joined: Dec 2018
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Sorry can't help much with PIWIS, but that would be nice to know also. I suppose you're trying to stick with the tires that you already have? I believe you may have meant .5% but I'm seeing the 255/35/19 front and 305/30/19 deviates .8% which is definitely too much for the system.
I used this tire calculator and on the tire comparison tab....https://tiresize.com/calculator/
I run 21in wheels with .4% and no issues on the 970. I have 265/35/21 and 315/30/21
I haven't come across 18in but I think I'm seeing that they're available, 245/50/18 and 275/45/18 are at .4%
Stock 19in tires sizes are ~0%, 255/45/19 and 285/40/19
The stock 20s rear is 295/35/20 fronts at 255/40/20 are at .4% also.
The recommended 20in snow tires are narrower tires in the rear at 285/35/20 and is at(negative) -.4%, same 255/40 in the front. I ran these before and no issues.
Your observation 3 of 265/40/19 and 305/35/19 is at .4%
I would say +/- .4% is ideal and probably a good chance that what you said of +/- .5% is set in PIWIS somewhere where that extra +/- .1% to account for tire treadwear. It would be really interesting to see if .7% or .8% or whatever can be set in PIWIS, but I can imagine it may be complicated Ż\\\_(ツ)\_/Ż
Just occurred to me that the 19in is ~0% and the 20in are +/- .4%, I wonder if you set your wheel size to 20in in the instrument cluster wheel settings regardless that you're running 19in to see if it's less sensitive to the difference at .8%
I used this tire calculator and on the tire comparison tab....https://tiresize.com/calculator/
I run 21in wheels with .4% and no issues on the 970. I have 265/35/21 and 315/30/21
I haven't come across 18in but I think I'm seeing that they're available, 245/50/18 and 275/45/18 are at .4%
Stock 19in tires sizes are ~0%, 255/45/19 and 285/40/19
The stock 20s rear is 295/35/20 fronts at 255/40/20 are at .4% also.
The recommended 20in snow tires are narrower tires in the rear at 285/35/20 and is at(negative) -.4%, same 255/40 in the front. I ran these before and no issues.
Your observation 3 of 265/40/19 and 305/35/19 is at .4%
I would say +/- .4% is ideal and probably a good chance that what you said of +/- .5% is set in PIWIS somewhere where that extra +/- .1% to account for tire treadwear. It would be really interesting to see if .7% or .8% or whatever can be set in PIWIS, but I can imagine it may be complicated Ż\\\_(ツ)\_/Ż
Just occurred to me that the 19in is ~0% and the 20in are +/- .4%, I wonder if you set your wheel size to 20in in the instrument cluster wheel settings regardless that you're running 19in to see if it's less sensitive to the difference at .8%
Last edited by Deezflip; Mar 9, 2026 at 04:09 AM.
