HD2pct

Yes, I know the proper size for 20s is 275/40. But I bought a set of 20" wheels that "came with" new Yokohama 275/45s. I think they may look a little better than the 40s and perhaps ride just a tad better as well.

I am a believer in using recommended everything, but I have these and the other side of me says that using them will not result in any spectacular crashes or gnarly mechanical failures.

Opinions?

Grizz

As long as you bought 4 you should be ok, because if you mix and match sizes then it can mess with your PSM and that would not be good. Also, your speedometer will be off unless you can select the correct size in the instrument cluster, I cant remember whether that size is available or not. Other than that, since it is such a small diameter difference, I wouldnt worry about it until you get new ones and then I would definitely go back to the 40 series. Also, if they arent "N" rated Porsche didnt approve them, so keep that in mind if you drive it really hard.

HD2pct

Thanks Grizz - I wouldn't mix sizes. I don't think you can specify tire size within the menu system, unless possibly with TPMS, which I am not using. I realize my speedometer will be off a little, but OTOH I have never had a car whose speedometer was dead on. In most cases they were optimistic, so in this case, the change may actually "dial it in".

No these tires are not N-rated. Not many are. I don't drive it hard per se, but as you say, I am all for going back to '40s when the time comes.

LVDell

Actually, NONE are in that size......only available in the size designed for the car. And running the wrong size could actually bring all kinds of havoc on your braking system etc since you have altered the RD of the tire. There is about a 4% change in RD which equates to about 26 more revol,utions per mile and a speedo that will read several mph slower than you are actually going. The difference b/t the 45 and 40 series here is not nearly as bad as it could be but personally I would have run the incorrect size tire.

HD2pct

LV, I do not see how this "could actually bring all kinds of havoc on your braking system etc".

If you can provide an engineering explanation for this statement that considers the design of the specifcially affected system(s), I am all ears. This is actually what I am looking for. My guess is that there might be some on this forum that know, thus the post.

That said, all things being equal, I would run 40s. But since this isn't the case and I am sitting here with mounted 45s, I have a decision to make. Yes, I do understand circumferencial considerations. They are minor.

LVDell

No I am not an engineer so you will NOT get an engineering explanation from me. However, changing the RD of your tire will result in a different revs per mile on the tire and for one your ABS/TC/ Transmission system(s) are based on the OEM sizes. Changing that can lead to them not functioning properly. You might be able to get away with it as long as all 4 tires are the same size and the RD difference is not significant. This variance differs between manuafacturers and I have seen it as low as 2% and as high as 5%. Your current setup is in the 4% range.

Maybe I should have selected a different word other than havoc.

Hopefully a technical engineer will give you a scientific explanation.

HD2pct

Dell - I guess practical experience might also be good - I figured someone would be running these and would give a first-hand account.

Looking at Tire Rack specs, from a diameter standpoint, we may be talking about a 3% difference. At the end of the day, I would have to put out that much more torque to overcome vehicle dynamics (straight line accel and braking). Obviously there are systems on the car that would be involved in that, but the question is whether it makes any practical difference.

I have seen people do what appears to me to be serious pimping out of their Cayenne as applies to wheels and tires. That isn't my style as it isn't yours, but maybe they can give some horror stories if there are any.

Or maybe when I receive them this weekend I will chuck them in a lathe and turn them down to approx 28.8" in diameter and we'd be all set! I might then only get 1k on them after that but it's better than tossing them!

LVDell

I think those that have run the abnormal sizes need to chime in.

charlieXS

the 275/45-20 is the same height as a 285/35-22 which TONS of people around here run... yes it hurts acceleration and is more unsprung weight but obviously noone's cayenne explodes or anything lol.

Grizz

Seriously, switch them over when you wear them out, but until then dont worry about it. There is no problem with the braking system. ABS will still work the same since it looks at tire slip percentages, not revs. If it looked at revs, how would it work at all speeds?

HD2pct

Yes, I agree - My feeling is that if they affect anything it would be the transmission control program and probably imperceptably, especially since it's a little flakey in the first place as we all know.

ABS, no, PSM, no. Speedo reading, yes but come on.

LVDell

Hey Einstein, revs per mile in constant regardless of speed. Unless you figured out the whole space time continuum thing.

By the way, I never said it would NOT work. I just said that he needed to make sure and stay within tolerance which he is on the margin with anyway. And if they are worn down a bit anyway he should be just fine. Hence, the reason I deferred to those with EXPERIENCE in abnormal sizes.

HD2pct

I do appreciate everyone's input and we can probably all agree:

Does if affect stuff - yeah, in theory

Would I notice - no

Would the thing blow up - no

All things being equal, is it a sounder decision to stay within the mfrs spec - yeah

Grizz

HD2pct- Maybe it will affect the trans, but considering the adaptability of the transmission control modules in these Cayennes, I think after a while it will adapt right back to your driving habits eliminating any shifting variations from the different diameters.

LVDell- I am assuming that you were talking about engine revs, and not tire revs, because then yes, they would be constant. I dont know why you would be talking about the engine though, considering that has nothing to do with this thread. Either way, I wasnt trying to insult, so back off, I am just here trying to help out fellow members...I kind of figured you would be a little more enlightened since you have been here a while.

If I assume wrong and you were talking about tire revs, then WOW, you really need to rethink things.

LVDell

Grizz, you really need to go back and re-read your math text. What part of revolutions "per" mile do you not understand. In this case the tire with the larger RD (rolling diameter) has a constant "per" mile based on it's size. So no matter what the hell speed you travel at it will ALWAYS have that constant "per" mile number as its revs. Not sure how you missed that but then again, I still don't understand your analogy of engine revs as you stated they would be constant. Maybe you care to explain that one. Remember, you are the one that stated "If it looked at revs, how would it work at all speeds?". Think about it, no matter what speed you travel, the revs per mile is ALWAYS the same.