Back Tyres

Ok you clever people, I need your advise on rear tyres. My C2 Targa currently has 285/30/18 on the rear and I would like to change to 285/ 40 /18 to reduce the road noise through the tyres and as an aside I think the rears look too skinny for the cars wheel arch. Question? can I do it and what inpact will it make on the speedo or perhaps sat/nav etc? I did try to ask my OPC but on a Saturday all I got was a very pretty young lady who could not understand what I wanted to do.

This will calculate for you Michael http://www.kouki.co.uk/utilities/visual-tyre-size-calculator Higher profile tyres will cause speedo to underread. (9.1% on the sizes you quote) No effect on Sat nav? Don't know whether it can be done, sorry!

Hi Alan Thank you for the info, the site is excellent. As you said my speedo would be out by 9.1% so my next question is can the speedo be adjusted in some way by 9.1% on a 996? If not I may reconsider fitting 17" wheels with higher profile tyres as per the hand book.

You can change the wheel size and tyre width/height ratio in the PCM settings, but not in the instrument cluster. Speedos tend to over-read, so fitting larger diameter tyres would probably make it under-read. I'd be a bit more concerned by what it might do to the handling, and you might have a problem obtaining an N rated tyre in the profile you are looking for. If you have a warranty, that might be an issue too....... I'd go for the 17" wheels if I were you.

Michael, There are a number of issues here ... 1) I think you'll find that "road noise" will be influenced by type of tyre, more than sidewall size. e.g. different makes with different patterns and compounds will have a greater impact than changing from a 30 to 40% ratio of the same brand. (Although only vaguely connected with Porsche I had some new Goodyear Excellence tyres put on our Golf TDi recently and they're so much quieter - especially on that light coloured concretey road surface - than the NCT's that were on it previously) 2) The sidewall size _will_ have an impact on firmness of ride, due to the additional 'suspension' provided by tyre flex. This is one reason I went for 17" on my 996, rather than 18. 3) I'd be very wary of attempting to put a 40 profile tyre on an 18" rim. That would increase the overall diameter and, as there is not a lot of room under the wheel arches, you _could_ be in danger of fouling the tyre against the bodywork when the suspension is fully compressed. And you wouldn't want that happening when (say) hitting a dip on an autobahn at 200km/h+ on a bend, for example. 4) As Richard pointed out, for handling reasons, it would probably be a disaster changing just the rears and not the fronts - the whole car would then have a nose down attitude due to being higher at the back. It could even invalidate insurance or worse. So, if, as you say, it is road noise you want to reduce, try a different brand of tyre. Others on here probably have more experience at which are the noisiest and quietest tyres - I have only used, but been very pleased with, my Michelin Pilot Sports.

Thanks for the info. I agree with much of what has been said but I should add that my brother has a 996 coupe with 997 wheels fitted to it which have 285/40/18 on the rear, his car is much quieter than mine and more comfortable over bumps.