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I've read about this purported "3% tolerance", but I've never seen documented proof that there's an issue here. I certainly don't believe it would be like a light-switch, where below 3% you are fine but above 3% you are screwed.
So I'm not entirely buying the statement that he's at risk of his ABS and PSM systems throwing errors. He's certainly at risk of the tires rubbing at full compression or full extension - that would be my only real concern here.
From a purely selfish standpoint, I'm glad that @Jeffrey Behr did what he did. My goal for my CGTS4.0 has always been to step up from 265/35-20 to 275/35-20 in the rear (and from 235/35-20 to 245/35/20 in the front). But I might be able to take things one step further in the rear with 285/35-20s, which is a size that has many more tire choices than the 275/35-20 does. Of course, I really don't have any complaints with the CGTS stock sizes, as I've got 5 track days, one SCCA time trial, and a handful of AutoXs on the factory-stock PS4S tires. The rear tires will be down to the wear bars sometime next year. And I do rotate the tires side-to-side as all the tracks I drive are run clockwise.
When I switched to PSAS4s a few weeks ago, i went with 245/35-20 F and 295/30-20 R and have absolutely no issues. I couldn't stand the narrow stretched look of the Pirellis on the rear of my BGTS. BTW, I just noticed you are from Reading. I was up until a few years ago. I could hear Maple Grove from my house.
Last edited by GrandPaJohn; 12-13-2022 at 09:51 AM.
You can see from this readout below your speedo error at various speeds. You've put a tall, high profile 295 on the rear that is almost 3% taller than the 265/35 tire it replaced. The ABS system has a 3% tolerance built into it for different sized tires, and this size approaches that. As you get near this threshold, your ABS and PSM systems can begin to throw errors.
If you'd used a 295/30 tire instead, (see below comparison) you would have had virtually no speedo error, and your ABS system would be a lot happier.
I've read about this purported "3% tolerance", but I've never seen documented proof that there's an issue here. I certainly don't believe it would be like a light-switch, where below 3% you are fine but above 3% you are screwed.
So I'm not entirely buying the statement that he's at risk of his ABS and PSM systems throwing errors. He's certainly at risk of the tires rubbing at full compression or full extension - that would be my only real concern here.
It's not "purported," was told to me, and shown documentation by two different friends, both chief techs at their respective dealerships, and Gold Meister rated by Porsche AG. I also didn't say it was a light switch, I said "As you get near this threshold, your ABS and PSM systems can begin to throw errors."
Believe what you like, but the first time you get ice-pedal at the end of a long straight on track because your tires are the wrong size, you'll pay attention. I've seen it a lot over the years.
Does that 3% tolerance relate to increasing or decreasing from oem size, or is it the difference between front and rear tires, regardless of size? For example, with the stock sizes of 235/35F and 265/35R the diameter difference front to rear is 2.9%. With the Spyder sizes of 245/35F and 295/30R the diameter difference is 0.7%. With the OP sizes of 255/35F and 295/35R the difference is 4.1%.
Does that 3% tolerance relate to increasing or decreasing from oem size, or is it the difference between front and rear tires, regardless of size? For example, with the stock sizes of 235/35F and 265/35R the diameter difference front to rear is 2.9%. With the Spyder sizes of 245/35F and 295/30R the diameter difference is 0.7%. With the OP sizes of 255/35F and 295/35R the difference is 4.1%.
It is related to increasing or decreasing from the OEM size. The difference or ratio between front and rear is not as critical, and if you stay within this delta for the sizes of front and rear vs OEM sizes, the front to rear ratio will be within spec.
Finally 'made' some Porsche-badge wheel centercaps for the Cayman
Never found Porsche caps to fit the Savini wheels, so stole the Porsche-badge plastic discs that I had found for the 2010 Cayman wheels.. Would have purchased a dozen more but they're no longer available.
Sorry about the dirty inner wheels; the carwash never touched them, and I was out of energy.
Last edited by Jeffrey Behr; 01-12-2023 at 06:45 PM.
New wheels--Savini BM12s--and Michelin Pilot Sport All-Season 4s, 255/35x20 fronts and 295/35x20 rears...
........ I think I got the wheel and tire widths right.
Please share your Savini wheel spec, what are their width and offset ? Thanks
Finally I received this info today; it wasn't on the purchase invoice.
20x8.5 Savini Black Di Forza BM12 Gloss Black w/ Machined Face (Concave) 5x130 +45 offset
20x10.5 Savini Black Di Forza BM12 Gloss Black w/ Machined Face (Concave) 5x130 +40 offset
Finally I received this info today; it wasn't on the purchase invoice.
20x8.5 Savini Black Di Forza BM12 Gloss Black w/ Machined Face (Concave) 5x130 +45 offset
20x10.5 Savini Black Di Forza BM12 Gloss Black w/ Machined Face (Concave) 5x130 +40 offset
Thanks for getting these numbers. Somewhat aggressive offset at the front (extended 12mm out) but it works and looks good 👍🏼 Rear is extended outward by 7mm so it's perfect.
Can you get a close up pic of front wheel from the front of the car to show how far the wheel extended past the fender (if any) please ?
Thanks for getting these numbers. Somewhat aggressive offset at the front (extended 12mm out) but it works and looks good 👍🏼 Rear is extended outward by 7mm so it's perfect.
Can you get a close up pic of front wheel from the front of the car to show how far the wheel extended past the fender (if any) please ?
These were shot about 50 feet away to NOT emphasis the size of the nose and with the lens zoomed to enlarge the car.
Jeffrey, you are such an awesome Rennlister. Thanks for the pix 😊
You are welcome, and TYVM.
FWIW, at 1000 miles, I still LOVE this thing...I simply have a smile on my face every time I drive it, which is every time I drive somewhere by myself.. It's tight, very responsive, and faster than a proverbial streak of lightning when I want it to be, and...sorry, MT folk...with the PDK it's never a chore to drive.. I remember when I had my '06 Cayman S with 6-speed, it turned into a 4-speed (1-2-4-6).
Last edited by Jeffrey Behr; 01-27-2023 at 02:31 AM.
12-13-2022, 09:49 AM
I certainly don't believe it would be like a light-switch, where below 3% you are fine but above 3% you are screwed.
