Tyre profile

C

chris_p

Member
I after some new rears for the zed, I've seen some tyres with a different profile have come up for sale, I can't imagine it would be a problem but figured I'd ask in here first!

Currently -
F 225/40/18
R 255/35/18

Proposed R 255/40/18

I would have thought they'd be better due to being the same profile as the front?

Thanks for any help!
 
The 255/40r18 is a rear ///M size, and the equivalent front is a 225/45r18. Despite the different profiles, the diameter & rolling radius are about the same.

The profile is the height of the tyrewall expressed as a percentage of the tyre width.

E.g. a 40 profile tyre on a 335 wide tyre would have a sidewall of 134mm, whereas a 40 profile tyre on a 225 wide tyre would only have a sidewall of 90mm. So completey different despite the ratio being the same.

In your case the difference between a 255/35 and 255/40 is 5% of 255 - or 12.75mm taller - which will have the affect of lengthening the gearing slightly, possibly dulling some acceleration, increasing unsprung & rotational mass, and finally, affecting your speedo.
 
That's one hell of a reply!

It'll fit alright then..
I was aware of the speedo inaccuracies but I'd of thought they'd be negligible?
 
chris_p said:
It'll fit alright then.
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The tyre will be taller, so it might scrub the arch liner on bumps. Only way to tell is to try it - although I think some on here have put Z4M wheels & tyres on non-Ms until they needed new tyres and then they tend to switch back to non-M sizes (but that could be due to the higher cost of M sized tyres) :P

chris_p said:
I was aware of the speedo inaccuracies but I'd of thought they'd be negligible?
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Depends what you mean by negligible as it's about a 4% error (compare this to a 1% error from a tyre will full tread to a tyre that needs replacing).

If you're speedo says you're doing an indicated 85mph on the motorway (which is usually safe enough to avoid a ticket as it's about 81mph real speed), the bigger tyre might add 4mph to your real speed when the speedo is showing the same 85mph - which means your actual 81mph becomes 84mph.

Not a huge difference, but it's up to you to decide whether you're happy with it or not.
 
My winter tyres are one profile larger than OEM (40 instead of 35) which results in the speedo reading the actual speed as checked by GPS.
 
Same experience here as Half Colin. (Not surprising really) Standard tyres read a bit high on speedo compared to GPS, winter tyres one size up are dead on.
 
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