Tyre Advice

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Thu 25 Mar 2004 16:46
Tyre Advice - midlifecrisis
My car is shod with 225/45 ZR 18 tyres. They are due for replacement and I've found this size to be quite rare (and expensive). It's been suggested that I can change the size to 225/40. Is this feasible and what effect will it have.
Thanks in advance.
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Thu 25 Mar 2004 16:55
Tyre Advice - Phoenicks
yes i would have thought this would be fine as it will only change the profile of the tyre (basically making it thinner).

Will probably make the ride worst, but will be increased tyre wall strength and therefore better grip in cornering.

What car is it for?
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Thu 25 Mar 2004 16:56
Tyre Advice - No Do$h
The 40 in the designation refers to the depth of the sidewall as a percentage of the width. In this case, a change from 225/45 to 225/40 will lead to a reduction in sidewall depth of 11.25mm.

Because you have a relatively large wheel diameter, this difference won\'t be particularly noticable visibly. You will find the following will change:

Acceleration. In theory, slightly quicker off the line
Speedo. Will over-read slightly as for each revolution of the wheel your car will have covered 35mm less ground. addendum. 70mm less ground. should have been 2 Pi r. Ooops!
Top speed. Marginal reduction.
Ride comfort. Less sidewall = harsher ride and more likelihood of tramlining and kerbing.

I would ring Micheldever for advice on this one if I were you. Whilst the change is possible, they may have had negative feedback specific to your car.

ND
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Thu 25 Mar 2004 16:59
Tyre Advice - No Do$h
Just a quick add-on. The performance/speedo differences I refer to above will be in the region of 3.5% so will be all but impossible to notice. The ride comfort will be the big decider.
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Thu 25 Mar 2004 17:11
Tyre Advice - LHM
I think No Dosh is spot on with his advice. Sounds like a relatively 'exotic' application, so better check with your insurers - just in case........

In Germany, I believe they are quite strict as regards this sort of thing.
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Thu 25 Mar 2004 19:07
Tyre Advice - midlifecrisis
It's purely a cost thing. The 45s are on average around £180/corner. However, there are some good internet deals on 40s. I wouldn't want to compromise safety for the sake of cash.
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Thu 25 Mar 2004 21:21
Tyre Advice - carayzee
Plenty of known brands on www.mytyres.co.uk at £135 e.g Potenzas or Conti Sports
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Thu 25 Mar 2004 23:56
Tyre Advice - Chad.R
Just to add to ND's and LHM's sound advice;

The 225/45 18 will have a diameter of around 660mm, the 225/40 will be around 637mm. So as ND says you will travel about 70mm less for each rotation, around 3.5%. Ideally you should try and keep this difference to a minimum (<2%?).

To reduce the aspect ratio to 40 but maintain the rolling radius you'll need to increase the width of the tyre to around 245 or better still 255 (653mm and 661mm respectively). These sizes may turn out to be more expensive than your originals and in anycase may well cause a problem with fouling the wheel arches etc.

Whatever you decide to do, I'd certainly run it by your insurers beforehand as it may be deemed to be a "modification".

Chad.
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