Choice of Tyre Size

[Octane]

My Cayenne Turbo 958 has 265/40 R21 105Y XL M0 summer tyres fitted (Conti Cross Contact UHP).

The list of expected tyres for the Cayenne Turbo from the manual is:
265/50 R19 110Y XL Wheel: 8.5 J x 19, RO 59
275/45 R20 110Y XL Wheel: 9 J x 20, RO 57
295/35 R21 107Y XL Wheel: 10 J x 21, RO 50

First, the garage I had it serviced at pointed out that they had homologation "M0” for Merc instead of "N*” for Porsche. I may be wrong but I thought this fairly inconsequential and not a safety / insurance problem. Anyway, digging deeper I then found there are only Porsche N tyres available in size 265/40 R21 for the lower load factor of 101, non-XL. 105 XL is the highest available but no Porsche approved ones… I think this means that 265/40 R21s are not intended to be fitted on a Cayenne, but on a smaller Porsche.

Initially I thought I must have narrower rims, but I found they are indeed marked 10Jx21 ET 50 (Turbo II design 7P5601025M), so they could have the standard 295/35 fitted. I thought the narrower tyre might be a bit better for efficiency and noise, and it looks and handles fine. I found that in this technical manual from Conti (page 54), 265/40 r21 is approved to be fitted on a 10J rim, although they have 9.5J highlighted as the default.

Aside from the tyre brand and homologation, my question is: does it matter that I’ve got 265/40, load factor 105 XL on the car or not? I don’t understand why the previous owner deviated from the 295s; doesn’t seem to be price since actually the 295s are marginally cheaper. These tyres have about 6mm tread left so would still last quite a while.

Thanks!


Key:

• In 110Y, 110 is the Load Factor, higher number supporting more weight per tyre
• XL is Extra Load. I’ve no idea what the difference between a higher Load Factor number is vs XL on a lower one.
• RO means Rim Offset (as per Porsche manual), ET is usually used instead. ET 50 means a 50mm axial offset.
• 9 J, 9 means rim width, J is the shape of the rim.
• ‘Homologation’ M0 = optimised for Merc, N0, N1, N2 = optimised for Porsche

 

[stevehaley]

see https://www.porsche.com/china/en/accessoriesandservice/porscheservice/vehicleinformation/tyreapproval/
there is a pdf list of summer tyres there which if I read it correctly you should have 295/35 R 21 107 Y XL available as yokohama Advan Sport, Pirelli P Zero or Michelin Lattitude Sport

Tyreleader have all three in stock
https://www.tyreleader.co.uk/car-tyres-295-35-21-107-Y/?sp%5B6%5D=on

I have the Yokohama on my Cayenne and am not impressed by wear or durability. Had one cut to the cord mid tread which has never happened to me b4 in 40yrs. Added to which I appear to have gone through 25% tread wear in 6k on mostly dual carriageway

 

[Octane]

Thanks Steve. Yes... I now realise the car *should* have 295/35 R21 107Y fitted as per the manual. My question is really, given the 265/40 is OK on the rims according to Continental, what is the issue / risk with leaving these tyres on? Since they still have maybe 10k miles left, its effectively £400 or so lost on their remaining lifetime if I chuck them.

For MOT it just says the load rating must support the max load per axle. That is 1,535kg (rear axle) in the manual -> 768kg / tyre. Load Index: 105 -> 925kg, 107 -> 975kg. So, it seems sufficient legally. Not sure if that is sufficient for insurance though?

I have the Yokohama.. Had one cut to the cord mid tread which has never happened to me b4 in 40yrs

Wow. Yeah, I've never had that. On a different car I've had Generals fail with bulging sidewalls.

I though I'd get get P Zeroes when I need to, the Cross-Contacts are quite noisy especially on the crap concrete part of the M25 where they resonate really badly...

 

[stevehaley]

ah didnt notice the size difference. The first thing is that your spedo will be underreading by 1% but this may be compensated by the fact the spedo probably overreads anyway.
i would guess it is also potentially an mot fail if they noticed.
https://www.setyres.com/news-article/will-your-tyres-fail-your-mot
You may also have issues if your transfer box failed and you tried to get porsche to cough up.
i think on a beast like the cayenne i would rectify it and try and sell the tyres on ebay or to a dealer. Having said that i used to run an xjs on smaller radius winter tyres for 2 mths every year with no issues for 10 years.

Thanks Steve. Yes... I now realise the car *should* have 295/35 R21 107Y fitted as per the manual. My question is really, given the 265/40 is OK on the rims according to Continental, what is the issue / risk with leaving these tyres on? Since they still have maybe 10k miles left, its effectively £400 or so lost on their remaining lifetime if I chuck them.

For MOT it just says the load rating must support the max load per axle. That is 1,535kg (rear axle) in the manual -> 768kg / tyre. Load Index: 105 -> 925kg, 107 -> 975kg. So, it seems sufficient legally. Not sure if that is sufficient for insurance though?

I have the Yokohama.. Had one cut to the cord mid tread which has never happened to me b4 in 40yrs

Wow. Yeah, I've never had that. On a different car I've had Generals fail with bulging sidewalls.

I though I'd get get P Zeroes when I need to, the Cross-Contacts are quite noisy especially on the crap concrete part of the M25 where they resonate really badly...

 

[stevehaley]

btw the cross contact arent approved for your car they are for a different model year which appears to matter to porsche.

 

[Octane]

OK thanks. I think I'll switch to the winter tyres/wheels fairly soon which will give some time to consider these. Could the tyre shop be at fault for supplying the wrong size tyres I wonder? Not only this, but one was mounted inside/out; I got that re-fitted and was advised it would have been an MOT fail. The MOT was done in Feb, so it probably had the winters on at that time which is why neither problem would have come up ...

I don't think the previous owner was someone who would have paid attention to all the technical details but certainly was not a cheapskate. Whilst I have a lot of paperwork for the car, I don't have the receipt for these tyres.

BTW, I don't mind what Porsche think per-se; it's (obviously) out of warrantee and has already had the T-case done. I care about safety/MOT and the insurance being valid. It seems that it is safe since the tyres are OK on the rims and the load index is adequate.

I haven't checked the winter wheels yet, they are in storage. Let's hope they are at least the right size and rating...

Thanks!

 

[stevehaley]

Winter tyre specs are there as well in a separate doc.
should be either 19 265/50 or 20” 275/45 dunlop winter sport 3d or scorpion ice and snow.

they seem to update the selection for each model year even if they are the same size.

i have michelin scorpion on my 19” wheels for a cayene diesel s 2017my. Was told i shouldnt use snow and ice which i think are a bit better. havent been used in anger as havent managed to get to the alps in the last two years.

whole thing is a complete pita as it means you cant use the latest winter tyres and they do get better each year. Unfortunately i still have a warranty running so it does matter.

i suspect you can play dumb and provide they are the right size and porsche approved it should be ok?

 

[Octane]

Thanks. Yes, given that in the manual it specifies the size and load/speed rating only, I expect for insurance and MOT the brand, model and Porsche N rating don't matter. I hope I find the winter ones are at least the right size and load rating! I'll check this first and go from there...

 

[Octane]

From the paperwork I've found the Winters seem to be the correct spec (275/45 R20 110V XL) but just aren't a decent brand (General Snow Grabber).

I've also found that the rolling circumference of 295/35 and 265/40 is close, so the speedo isn't that far off. Maybe within 1% is the official tolerance?

https://alloywheels.com/tyreCalculatorResults2
"FITMENT OK! The percentage change is 0.74% and the circumferece change is 5.5mm."

I also took advice from the AA and found that the MOT does not include checking the tyres match the vehicle spec, but just are identical and in good condition.

Update, I called my insurer and notified them that the tyres were 265/40 105XL instead of 295/35 107XL and they said "it didn't matter, they didn't look into this level of detail", they only seemed to think the condition was important. I was quite surprised, but there you are!

 

[euklid]

The person that put 40 profile knew what they were doing as 35 is way too low for an SUV that size.

But it's not bad though for comfort with 35 profile.

I need to change and thinking of putting 40 or 45 profile on my 295/35 21 rims.

I'm trying to find what will fit.

It's going to be more comfy.