RS 3.8 tyre sizes

[DSCBoy]

Now, I must admit, I'm not an expert when it comes to tyres with tread on...
Mostly because they don't tend to last long.

Anyway, the museum pieces on the "Infineon" let me down today, the inner sidewall cried enough on one of the rears so now I'm hunting down some replacements.

BUT currently I have
235/35 ZR 18 Y86 on the fronts and
285/30 ZR 18 N2's on the rear. All from the house of Pirelli.

Now, given those fronts are a bit rare and I've yet to find an online site that stocks all 4 tyres in those sizes from the same manufacturer I checked a bit online and I note that some sites suggest that 235/40s and 285/35's are the original tyres sizes for a Speedline shod 3.8 RS???

A) Anyone know if this is true
B) Does it make any real difference if I go with the latter tyres sizes?

The Pirellis are pretty "round-shoulder" tyres so sidewall as a percentage of tyre is probably smaller than a "square" shouldered tyre? Could this be the difference?

Help! I'm on a bl**dy spacesaver and it's hurting my street cred!

Rick

 

[DSCBoy]

While we're there...

Anything to stop me taking 235/40s on the front and 285/30s' on the rear as these seem to be the most available tyres and which I can get N-rated. Which I'm told is Porsche Homologated... N4's vs N2's??? Help?

 

[DSCBoy]

Talking to myself here, but just found a very good older discussion on Pistonheads where some bloke called Melv puts the World to rights on N-ratings. So Happy I've grasped that, just intrigued about tyre sizes now?
Cheers
Rick

 

[Talat]

You can work out the RR differences via:

http://www.club80-90syncro.co.uk/Syncro_website/TechnicalPages/TRC%20calculator.htm

I have the stock sizes in a book somewhere at home - will post them later.

 

[DSCBoy]

Appreciated Sir...
So going up from the 35 to the 40 aspect ratio adds 10mm to the rolling radius...
Doesn't seem toooo crucial.
Be interested in what you have as the original sizes.
Seems now that the sizes most commonly available are the 285/30 rears that I currently have and the 235/40's that are supposedly original fit on the front... can get N rated Michelin Pilot Sports for both ends then.

 

[Talat]

The stock sizes according to my book are 235/40/18 (Fr) and 285/35/18 (Rr). I'd be tempted to go 295/30 on the rear rather than 285/30 depending on price/availability.

 

[Melv]

According to info I have, slicks are 245 x 645/18 and 305 x 645/18!!

 

[DSCBoy]

More options so I'm just more confused now! Sounds like I can hang some wider rubber on the rims if needed...now to find out what I can get delivered tomorrow-like!
Thanks guys