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Hi there Mick, what you are referring to is a system (or principle) initially developed by Pirelli called the 'Plus One' system. Whereby you increase the size (usually width) of the tyres already fitted to the next size up.
I haven't done this on a Clio, but have done this on a few cars I've owned in the past.
The things to look out for are;
- Are you increasing the sidewall height of the tyre? If so, by how much?). This is calculated (in mm) by dividing the width of the tyre (e.g. 195mm) by the percentage aspect ratio (in your case, 50%) which gives a sidewall height of 97.5mm. Compare this with what you already have in place.
- If the difference isn't too great (+/- 4mm) then the effect on the speedo/gearing is minimal, anything outside that range will have the effect of (if greater)increasing the gearing & making the speedo under-read or the opposite (if less).
- The next consideration is clearance within the arches & splashguards. Before considering any changes, you should have a good feel around the tyres (front & rear, remember - and put some people in the rear seat as well) and with the tyres on full lock in both directions. This is to ensure that any increase in sidewall height will not cause fouling of the tyres under any normal driving circumstance. Plus, of course, you will be increasing the width of the tyre, so any increased width (i.e. 10mm) will be split evenly by (+ 5mm) on the inside of the wheel and the outside.
- You must also ensure that the tyres will not protrude from the wheelarch - this is illegal.
Hopefully, I haven't put you off, and if it all works out, you will enjoy all the benefits from having a greater contact area with the road. Regarding tyre choice, I would definitely recommend a directional tread pattern (more consistent wet weather performance over its life), buy the 'best' brand you can afford, and shop around (online as well) beforehand.
This is definitely a case of measure twice and cut once.
All the best,
This job should never be complete - the day I think I've done it all, is the day I resign.
Currently in VelSatisfied's garage:
2005 FIAT Ducato 2.8JTD LWB Gran Volume, 2000 Kawasaki ZZR 1100 D7 'fullpower' in black + full GIVI & KAPPA luggage, 2007 MB R-Class 320 CDi Sport LWB uprated by Brabus to 300Bhp don't know if there's a 155mph limiter - will be fun finding out! 2004 Mercedes Vaneo 1.7 CDi Ambiente 7-seater