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What are the variable Tyre Sizes for my 17" wheels


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  • Rousey changed the title to What are the variable Tyre Sizes for my 17" wheels

Hello Alan

We came across a similar situation with our XJS i.e.differing tyre sizes.       By reducing the rolling diameter from 245 to 235 you are leaning towards speedo inaccuracy and auto gearbox change point variation.   There is a formula to calculate % variation and the parameters are +/- 2%.      You wouild be reducing the rolling diameter from 45% of 245 to 45% of 235.

It would appear that to change to 235 tyres you would need to go for 235/50 rather than 235/45 on your 17" wheels.

However, we should suggest a call to your local branch of ProTyre (Chemix)....they are usually very helpful and can calculate the formula exactly.

As an example, we have stuck with the originals on our S-type facelift, but on our XJS we changed from OE Pirelli P600 235/60 (difficult to source and, if found, priced at a minimum of £215 per tyre) to Falken 225/60.........the reduction in rolling diameter (60% of 225 as against 60% of 235) resulted in a variance of -1.9%, so just within the limits.  But with a saving of around £115 per tyre (with more modern tyre technology, more rubber on road etc).

Hope this helps, but do have a word with ProTyre, then you should have a definitive answer.   Whatever you do, don't go for budget tyres....enough said!

As always, happy growling

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Hi Carole

Thanks for your responses but I am now confused !!

Most websites when I plug in my vehicle registration, quote 235/50R17 as the correct size (along with 245/45ZR17 which I have currently fitted) as does  my owners handbook  -   have Protyre said this size is NOT permissible ?




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