Pirelli don't make 235/35/18's anymore. So I fitted 245/35/18's on the front of my Roadster. The overal diameter is only 4mm larger. Would that damage the viscous coupling? Since the tyres have 8mm worth of thread depth, I can't see how 4mm would make a difference when there will be a 8mm difference as the tyres wear out.

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'97 Roadster
Australia

You should be OK, that is a very small difference. The only way to really know is to drive the car and make sure you don't hear the differentials start to whine and make noise due to excessive heat build up.

The best thing to do is figure out what the tire size's "RPM" is and then find a tire that matches it as close as possible.

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Standard fronts for pre 98 roadsters was 235/40/17. You upgraded your rims to 18?

-mick

Yes, I have '99MY wheels.

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'97 Roadster
Australia

The upgrade throws an additional concern into this but, IIRC, both MY98 and 99 (18 inch rims) listed both 235/35/18 and 245/35/18 as acceptable tire sizes. Not something I can double check until the weekend tho.

-mick

Pirelli still makes 235/35/18 in Pzero Assimetrico. I just bought 2 tires made in 2012
I agree they're pretty hard to find !

Now on my SV MY99, 245 is not listed as accepable tire size so I don't have a choice. It's surprising it's not the same for the roadster.

Be careful, since if you have an accident, insurance compagnies can point out that you have the wrong tire size...

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Car history :
2004 Renault Clio 1.6 (2004 - 2008)
1998 Porsche 996 C2 (2008 - 2010)
2007 Porsche 997 GT3 mk1 (2010 - present)
1999 Lamborghini Diablo SV (2012 - present)

Or just pull your front drive shaft.

Haha !

Makes me think of people who have an accident on a track and then tow the car on a public road for the insurance...

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Car history :
2004 Renault Clio 1.6 (2004 - 2008)
1998 Porsche 996 C2 (2008 - 2010)
2007 Porsche 997 GT3 mk1 (2010 - present)
1999 Lamborghini Diablo SV (2012 - present)

On the 6.0 SE the Lamborghini factory list both 235 and 245 as correct front tires.
May I ask where you got your 235's from?
Good to know if I need new tires.

I got them from Tyremark in the UK. They had two rears and one front.
And the other one, it's my mechanic who ordered it directly to the Pirelli plant in Turkey where these tires are produced.

Pirelli told me they should make a new batch around Christmas, for both 335/30/18 and 235/35/18. Then they will be more easily available in 2013 I guess.

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Car history :
2004 Renault Clio 1.6 (2004 - 2008)
1998 Porsche 996 C2 (2008 - 2010)
2007 Porsche 997 GT3 mk1 (2010 - present)
1999 Lamborghini Diablo SV (2012 - present)