My S Type is currently fitted with 245/45R17 tyres which need replacing.

Would 235/45R17 tyres be acceptable on these 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

hi

go to this link and see what the difference is https://tiresize.com/comparison/

cheers

Joe

Hello again Alan

Bye the bye, had occasion to speak to our loccal Protyre today, so mentioned your issue.  Actually, you would be OK with your alternate choice, but not with 50 sidewall as we thought may be the case.

 

Good luck

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 ?

Regards

 

Alan 

hi

from the handbook 235-50-17 are oe fittment

245-45-17 are ok, but are a sportier drive more for handling, these will give a harsher ride

cheers

joe