Max tire diameter, 996 NB?

Have searched and not found a direct answer to this. The reason for asking the question is that I need to replace my rear tires soon and the best replacement are PS2s which will cost about $1k installed (current tires front and rear are Pilot Sports, 6 years old in front and 5 years old in rear). Current sizes are 225/40-18, 285/30-18, diameter 25.1" (calculated). They are on Carrera lightweight 8" and 10" rims. Pilot Super Sports, widely considered a superior tire, would cost just a little more for a full set, but aren't available in my rear size. The rears are available in a 35 aspect ratio, but that would increase diameter 0.8" (3.3%) over the fronts which is likely to cause PSM/ABS issues. However, if I move up to a 45 aspect ratio in the fronts, the Super Sports are available in equal diameters of 25.9" front and rear. Therefore the question: is a 25.9" diameter tire a problem? Tire widths are stock. I measure maybe 0.6 - 0.7" of room above the inside edge of the tire to the front strut flange, but do not know how or whether a taller tire might rub on the fender under hard cornering, or whether tire deformation under lateral forces might push it against the strut flange. The car has the X74 suspension so is low to begin with. (Yes, these would add 0.4" to the ground clearance.)

I have 285x30x19 Bridgestone on the rear and I had tried to put 305s , but too wide. Since you have 18s, you might be able to do the 305s. Check with you local tire dealer or check with Eli over at Wheel Dynamics. The majority of his customers are Porsches He is very nice and very very knowledgeable

Not sure if the link will work: http://www.1010tires.com/Tools/Tire-...0R18/295-30R19 I am running 295/30/19 rears on my 40th anniversary with H&R springs - as low or lower than your picture. This size is almost 26" and fits ok. I have a very slight rubbing issue at the front at full lock and rears will rub the fender at full compression but due to the width and not the diameter hope this helps

235f/285r on 8 and 10 inch rims. No rubbing, but I'm at the max, given my suspension set up.

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What size are your front tires on your 18" wheels

2004 40th Anniversary 235/40/18 front 285/30/18 rear on Victor Innsbruck rims 8.5"x18 front 10.5" rear w/ 55mm offsets. I had 295/30/18 on OEM 10x18 rims and they fit fine but with the x74 unless you roll fenders "may" be tight so I backed down to 285.

235/40/18 front....rubs when you turn wheel all the way. Rear 295/30/18 no probs.

It would depend on offset, camber, rolled fenders, if you're using coilovers, etc. I'll be running 245 front and 295 rears on my NB starting next week.

Im also running 295/30/18 on rear and 235/40/18 on front with PSS9 coilovers. No rubbing issues

I've run with 235/40/18 front and 295/30/18 rears with the x74 on my '02 with no issues. My wheels are GT3 size and offsets and that's the key IMHO.

So far interesting data but it appears most people are more interested in wider tires. But this question is specifically about standard width but taller aspect ratio, i.e. maximum diameter given standard width. And maybe with a touch more emphasis on the fronts than the rears. The benefit is that you can get PSS's all around on stock 18" wheels and still match diameters front to rear. 6ta1's experience with 295/30-19 is the closest answer and given that they fit even though they are wider and a touch taller than what I'm considering is good news. 6ta1, what size are you running on the fronts?

Sorry...forgot to mention I have 10mm spacers. oopps

Mike.....personally I would not go for the taller tire. Taller sidewall means not as stiff. The sizes you are looking at are dimensionally fine. If you can go wider but not taller would be better. I have 35's on the rear which are taller than 30's but the wider contact patch that the 35's (295) gave me sealed my choice. If PSS came in a 295/30 that would be the ticket. I know I'm contradicting myself but what I have works well for me.

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my fronts are 235/35/19 which doesn't help you much (25.5" diameter according to 1010tires) as I mentioned earlier the fronts rub at full lock but never in "driving" condition

Yow, I ran into this posting on the Michelin forum dated 11/21/2012: "We are currently working with Porsche on their latest models with the Michelin Pilot Super Sport and not on the Porsche 911 type 997 models. Therefore we will continue with our current technical approvals on the Porsche 911 type 997 models for the foreseeable future - that is the N specification Michelin Pilot Sport PS2's, and the N specification Michelin Pilot Sport Cup+ tyres for certain applications. We do not expect to have a Porsche approved Pilot Super Sport fitment for the 911 type 997 models until at least 2014." http://motorsport.michelin.co.uk/forum/viewtopic.php?f=8&t=18#p307 So that probably makes it even less likely the 996 will be supported in the next couple of years. So either you go with odd sizes, or use a different tire.

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Maybe a typo (as in the other thread about x74 first impressions)? PSS doesn't come in 295/35-18, confirmed both at the Michelin and Tire Rack sites. But I think I'm leaning in your direction: 235/40-18 (25.4" diam), 285/35-18 (25.9" diam). Difference is 2% which should be OK. And tread wear from 10/32 to 2/32 is 0.5" in diameter (another 2%) so by the time the rears are at tread limit the diameter difference will be 0.25". New rears with half-worn fronts might be interesting though, with a 3% diam difference. Maybe by then there will be a 295/30-18.

I think you will be fine with PSS at 235/40/18 and 285/35/18. I am running 235/35/19 and 305/30/19 with no rubbing, on my non lowered 996 40th. In fact, I've ordered 235/40/18 and 295/35/18 BFG Rivals As 6ta1 suggested, the front tire diameter will be the limiting factor at full lock. The rears can go very tall as my fitment suggests.

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balefire, I am interested in your feedback. Have you tried any other size for the rears? The 295/30 or 305/30 are quite larger (in diameter) than our 235/35 fronts and I have been wondering if that makes the PSM act too quickly (thinking the rears start to spin earlier than they really do since they are like 5% taller than stock) I was thinking about going 275/30/19 for the rears which would solve the diameter issue as well as balance better the handling (I have not enough oversteer on the dry and too much on the wet)

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I'm running wb offset wheel with 295/30, stock height (for now) and no rubbing. At stock height, I wouldn't be concerned upping to a 35 series tire. You might get better mpg (.5mpg) with the slightly taller ratio. +1

Also, look around at different tire manufacturers. Michelin isn't the end all.

I asked Tire Rack what they would recommend and the response is: Yow that's quite a different approach from what you all have recommended. According to Tire Rack's web site, the diameter of these is 25.3" (rear), 25.1" (front). 275/35-18 are also available, diameter 25.6". Again I'm not after the absolute maximum possible grip by pushing width. I'd rather get the PSSs in whatever size makes the most sense. I like the idea of keeping with the stock front sizes to prevent rub.

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yupppp...typo. Go get them they are great.

265/35 is the winter tire size recommended by Porsche on both the nb & wb cars. It may look stretched on your wheel. why not go with a different tire?

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6ta1. I've tried 295/30/19, 305/30/19. I've never had issues with PSM, perhaps because I have a C2. I would consider changing alignment (max out camber front, 0 toe front, decrease toe rear, decrease camber rear) to reduce understeer before changing tires. I would consider increasing my front tire width before I decrease the rear tire to reduce understeer.

With your lowered x74 and keeping your rims, as others have said, the fronts determine max. I went through the same quandry you are in now, and just decided to spend a little more, and go with the right sized PS2s, which were under $1400 installed at costco, with road hazard, m&b, and N2 fill. I also use 7.5 mm spacers all.aroind from FVD. Some people, like me, just prefer Michelin tires because of past experiences, and afew $100 extra is worth it. Almost a year on the PS2 s and zero regrets. Alignment is the key to good wear. In my case, at or less than 5k miles ayear, they will last a very long time. Fun level and handling/ride are far more important than wear to me. What kind of miles per year do you expext to run?

Some of the bigger Ive seen. There are some NB 996's running 315's.

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IIRC, 265 is the standard stock size that came on '99-01 models. Starting in '02, they started coming with the 285 width as standard.

265 was standard on 17's, 285 was always with optional 18s, I thought for all years.

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Your thinking about the mk2 models. The mk1 models always came with 265 standard for both winter or summer setups. Look at mk1 manual or tirerack site.

Great choice! This is the best tire I have had.

All right, I ordered 235/40-18 and 285/35-18 after talking to my mechanic who figured the less-than-2% diameter difference, and 3.5% max diameter increase in the rear, should not be an issue, and he's very conservative about going outside the Porsche recommendations unless he has a lot of direct experience otherwise. Cody from Tire Rack called literally within 5 minutes of my placing the order and wanted to confirm the upsizing on both, and said he has seen this setup on this car and it seems to work.

Not to split hairs here, as the difference you are talking about is marginal, but it is something to consider...increasing the rear tire diameter will result in a slight loss of torque across the powerband by effectively lowering the final drive ratio (reduced mechanical advantage). Also beware that your speedo will read slightly slower than you are actually traveling. But on the plus side, your top speed may increase!

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I measured (with a phone app using GPS) actual vs. indicated speed on the speedo after I've read a few postings that didn't have a definite conclusion. The result is that Porsche appears to have designed the speedos (both digital and analog) to *always* read 4 MPH high at all speeds approx. 25 MPH and up. When it says 30 you are going 26; when it says 101 you are going 97, at least with my tires right now. So the higher diameter tires (by about 3% worst case) will actually partially correct this error. Apparently this speedo error is due to a German (?) law that requires that speedos never ever show a speed lower than the actual speed, so manufacturers build them to always show higher speeds. I doubt I'll notice the 3% torque reduction. OTOH it lets me stay in the lower gears a little longer, so on the whole it may be close to a wash. But I'm counting more on traction for my "fun factor" anyway.

Just to follow up and conclude this thread with the additional data point, I've had the 235/40-18 and 285/35-18 on the stock Carrera lightweight 18 rims for 8 months now and have had no problems at all. One good track day so far without any issues. Yes, the tires are awesome. I immediately felt the increase in grip and love how they respond to steering input. It was a surprisingly big step up from the PS2s previously on the car. I am guessing from feeling around that the PO roll the front fenders so this might be necessary but I can't know for sure.