I am about to renew one pair of tyres on my 957 and would like to go up a profile size (for a couple of reasons) and wondered if I am going to encounter 'wind-up'..problems
Existing tyres are 275/40/20 and I would like to go up to 275/45/20 . This equates to a difference of about 3.5% in the rolling circumference.
Any comments guys?

 

I'd really like to know what the max allowable difference is, too.

I asked a similar question in this thread
https://rennlist.com/forums/cayenne-...l#post15053867

Where I was wondering only about using a single (vs. pair in your case) mis-sized tire. In my case it was the collapsible spare tire vs. my 22" wheels.

Never got a believable quantitative answer (such as >2% is definitely bad, <0.5% is definitely OK, in between depends ...), but there is some good info in that thread.

Then, coincidentally, about a month after asking that question, I got a flat tire. No problems while driving on the collapsible spare. And while it was installed, I did some rolling circumference tests in front of my house - counting revolutions on all 4 wheels when rolling forward in a straight line on flat road. Turns out the difference on the spare was 2.3% (vs. the 1.1% predicted by the standard formula). And no warning lights or anything, but the spare was only on for a few miles, and I kept the speed below 50 mph (not the law, but just a good idea for spare tires).

The basic concerns for me (not caring about speedo or racing issues) are just if it would damage the differentials or transfer case, or confuse the traction control / ABS. I know that at some point the difference will be too big, resulting in damage under normal conditions, and below some point, you'll be safe from damage under normal conditions. Still wondering if the thresholds are quantified by Porsche. Any official Porsche info I've seen says stuff like, "if there is a problem with any tire, go to your Porsche dealer for a set of new tires."

 

It's not recommended for AWD. Just replace all 4 with the same size. As a bonus, I am pretty sure you can run even bigger set (275/50/20)

 

If you ever press the differential lock button with 2 different size tires on the vehicle, you could suffer significant problems, even if the diff lock button gets pressed by accident (kid, dog, etc.).
Have you looked to see if there is a 45 or 50 series with the same circumference as the 40 series?
Does a 265 - 45 - 20 have the same circumference as a 275-40-20?

If you are traveling 100 mph, one axle would be spinning at the "right" speed, while the other axle would be traveling either 96.5 mph or 103.5 mph depending on where the speedometer is connected. Even at 50 mph, one axle is going to be trying to travel 1.75 miles per hour faster/slower than the other. That's like asking the transfer case to absorb 1540 feet of over-spinning or under-spinning axle every 10 minutes. Seems like a lot of transfer case slippage.
All of the rotational speed difference will be concentrated in the transfer case. I wouldn't do it because of that.

 

Unless the T/C is locked up, you won't see 'wind up'. It takes 2 pushes of the off road button, and you have to be in neutral for the first one (into low range). If you accidentally end up in low, you will know it.

However, the T/C is clutched. Running a tire difference that big will mean you are slipping the clutches a lot. I don't know how much of that they can take. I don't know the consequences of burning them out.

At some point, the difference will confuse the ABS & PSM. There have been a couple posts from people with different sized tires, who had issues when the smaller ones were underinflated, or the big ones overinflated. If they ran the bigs soft and the littles hard, they didn't have the issues.

I wouldn't do this myself. If you want different sized tires, replace all 4.

 

There is enough difference. The car will NOT respond well to running these two different sizes simultaneously. I have done it with exactly these sizes (I have two sets of wheels withe different sizes: 275/40 were on the back and 275/45 were on the front) when I was doing a quick run around the block after a repair that required me to remove the front wheels. I swapped the fronts and had not thought to swap the rears before the test drive. The car drove fine until I got a few blocks from my house, then the car freaked out. You need to replace all 4 at once unless you are going to drive it a very short distance.

 

While I can understand different sizes will affect the car behavior what happens if you use same size but different brand sets? I have read it will not work with the PSM but I cannot understand why since there is no spinning differences.
I had the front set replaced after one tire burst and I put Vredesteins and now that the rears need replacement there is no stock of these and expected to be available in more that one month which I can't wait for. Should I go for a any other brand with the same size?

 

you should be fine with different brand but same performance category and size. I'd like to read your source on why PSM won't work in your scenario.
Only concerns I can think of are:
huge differences in actual size between manufacturers
big difference in performance specs (such as putting high performance tires in one axle, then putting big tread all terrain in other axle)
Handling differences between the tires, especially at the limit on a racetrack. Doesn't apply for regular driving.

 

I do agree with you but I quickly checked again the forums and I found one post in the other renn site where replacing the front tires to match the rears sorted a clunking issue, referening as to a different pattern as the culprit and another user reporting the same case right under. I`m pretty sure I went through other threads in the past stating something similiar and how accurate and sensitive the electronics are in these regards. That makes me wonder...
At least I`ll be out of the county almost a month and that should give the shop some time to stock back but I´d like to know if mixing is fine anyway in case they don´t because waiting too long would lead to significant tread wear difference I am afraid.

 

I'm back from vacation and the Vredestein stock is expected in two months so I'll be putting a different brand set in the rears. Would anyone know by any chance if the Yokohama's Advan Sport V103B are as good as the Vredesteins? Other recommendations? I'm looking for something of the same quality and performance to avoid weird reactions from the PSM.

 

I am running these winter sets with no problems. Granted they are nearly identical rolling circumference.

 

Coming from Subarus my understanding is that the Cayenne AWD design has slightly more allowable difference between all wheels. That being said I would not run two different brands even if they are the same size/performance category etc. I would definitely not run two different profiles at once even if they "calculate" out to be the same. Manufacturer's sizes are not perfectly identical and differences in tread-wear, depth, pattern, inflation pressure front/rear etc could really exacerbate it. Thus the problems some others seem to have had.

Sure you might get away with it, but you will likely never know there is a problem until it becomes serious. I would not want to be the person who had to resolve a drivetrain clunk by fixing my tire sizes.

You can get away with using the inflatable spare (of a ~2% difference in size) because it is only 1 of 4, and the total difference would be within the system allowable spec.

These cars have open differentials in the front and rear axles, but the connection in the center (T-case) will still communicate (and need to slip or absorb) the total difference in number of rotations.

Presumably you could mitigate any differences in rotations front-rear by putting one brand/size on the left, and the other brand/size on the right. Making front/rear identical. It would be like you were going around a turn all the time. Unless the difference was large enough to error out the PSM, it should drive and not hurt the center/T-case. This would be very inadvisable due to possible differences in braking, handling, etc. Even if you do not drive hard, this could become evident in rain, low traction of any kind, or emergency situations.

So in my semi-educated opinion, I would say it is important to keep them all the same size, profile, and brand. I would even hesitate to run a big difference in wear.

The maximum allowable difference is posted somewhere on this forum or in the owner's manual. This is useful if you are replacing one or two tires when worn or damaged. As I said I believe it's pretty permissive, at least compared to Subaru (roughly double). However when you start mixing brands, models, profiles, it is almost impossible to tell using online calculators or the manufacturer's stated tire sizes. The only real way to test is to use the chalk or string methods. Seems like a lot of work to avoid doing the right thing and putting 4 of the same tire. The other way is to use your car as a guinea pig. Between the performance issues with having different tires, and the possibilities of catastrophic mechanical failure, I wouldn't risk it.

 

Are these your front and rear sizes for your winter set? Are they the same brand? What will you do when one axle (probably front) wears down faster, and it is time for rotation? Seems that any difference you have will be exacerbated by wear and the different inflation pressure front/rear. Surely you don't want to end up with the wider ones in the front? Seems that this could end up becoming a bigger issue. Or am I misunderstanding?

 

Here is an excerpt from page 286 in the owner's manual. I can't find anything about damage to differentials. I seem to remember seeing it somewhere written or posted, measured in 1/32"s.

This page states 30% is the maximum allowable difference in tread depth on an axle. If we assume a tire starting depth of 11/32", that would make the maximum allowable difference ~4.4/32. Which is in line with what I recall seeing elsewhere - double the Subaru spec of 2/32".

This seems to indicate that is the maximum allowable difference *on that axle* when replacing one tire. So is this the maximum for the entire system? Further down they state to replace at minimum both tires on an axle at a time, and that all should be identical. I don't see any big *WARNING DAMAGE TO DRIVETRAIN* text, but maybe I just am not looking in the right place.

 

Just a quick update on my experience. I decided to get all of my tyres changed at the same time to completely eliminate any confusion so made my fitting appointment. During the fitting they found that one of the tyres had a fault and with no more stock available, my only option was to come back the following day to have the last tyre replaced. This meant that I had to travel home on three new tyres (275/45/20) and one of my old tyres (270/40/20).

I was obviously quite cautious on the drive home but immediately my car freaked out and went into limp mode until I switched off PASM and continued home. Having the final (correct sized) tyre fitted the next day allowed everything to return to normal .