Different tyre profiles back and front

[pashe]

Hey folks

I have a 2005 987 Boxster with the 18" Boxster S Wheels. I ordered new rear tyres (Conti Sport 3s for the first time BTW) last week but made a mistake when ordering. The recommended rear tires for the standard Boxster with 18" wheels are:

265 40 18

and I ordered the Boxster S spec tires:

265 35 18

I have the recommended 225 40 18 tyres on the front, these are pretty new and don't need replacing.

Will I have any major issues if I fit the 265/35 tyres - or will this not make much difference? It's just going to be a real pain to return the tires and wait for replacements.

Thanks!

Paul

[acadian_dad]

Minor correction - at least according to the 2005_987_Technik doc from PAG (available on this site somewhere ... ;) ) - both the base and -S versions of the 987 should use - 235/40 ZR 18 and 265/40 ZR 18.

The 225/40-18 you have in front was an optional size for the 986 instead - and interestingly the matching setup in the back was 265/35-18 (what you ordered) ... So your full set of tires will now be a proper match for a 986 ... but not a 987.

Gut feel is that it will be OK - but why not run with factory specs ? A comment in the 987 Technik document says

The wheels of the new Boxster (987) series have been redesigned and

made larger. The larger wheel/tire dimensions have been specially

developed to further increase performance. The wheels have been

designed to appear flush with the vehicle’s body. For the first time, all

the wheels are produced by flow forming process. This is a light weight

construction technology which reduces weight by rolling out the wheel

rim.

[pashe]

Thanks acadian_dad

I returned the 265/35s and re-ordered the 265/40s. Just decided it would be better to stick to the factory spec, as you say.

cheers

[gregtachyon]

The main thing with tire sizes is the tire diameter. If the difference is 5 percent or more you could have problems with the ABS, PSM.

This was taken off the PCA website:

In a previous response, you stated that "A 5% or more wheel speed difference will cause a fault in the [ABS/PSM] system."

My question: is there any loss in performance below 5% as you approach the 5% fault limit?

The performance of the ABS/PSM is at 100% up to 4%. At the 5% and above then it would not operate correctly. The initial percentage is an acceptable buffer zone for the system. This is for tire wear, manufacturing tolerances etc.

[acadian_dad]

The main thing with tire sizes is the tire diameter. If the difference is 5 percent or more you could have problems with the ABS, PSM.

This was taken off the PCA website:

In a previous response, you stated that "A 5% or more wheel speed difference will cause a fault in the [ABS/PSM] system."

My question: is there any loss in performance below 5% as you approach the 5% fault limit?

The performance of the ABS/PSM is at 100% up to 4%. At the 5% and above then it would not operate correctly. The initial percentage is an acceptable buffer zone for the system. This is for tire wear, manufacturing tolerances etc.

I'd seen this post on PCA tech forum as well ... but it seems to me that what is critical is that the front to rear RATIO of circumferences (or diameter - same thing basically) is consistent within this error bound (lets just assume we aren't being silly and skewing left to right ...). Its by measuring relative rates of change of the wheel rotational velocity (ABS sensors) that the ABS/PSM ECU would get its input. Eg. reducing to the ridiculous, one could in theory run with 20% errors as long as front and back errors were matched ... right ?

I noticed that most of the R-compound sizes people are running with are about 1-2% off the OEM specs. This worried me at first but seems at least within this 1-2% range, people are not seeing issues.

Dave