tyre size increase

z4newbie2021 · Post by **z4newbie2021** » Thu Mar 18, 2021 2:22 pm

Hi,
I am thinking of changing my tyres to something with a bigger sidewall.
i have a 2.5 se running 135 style wheels 18 inch 225/40/ R18 fonts and 255/35/ R18 rears
planning to change to 225/45/ R18 fonts and 255/40/ R18 rears just for a better ride.
i know it will put the Speedo out by +3%
any advice would be appreciated.
cheers

brillomaster · Post by **brillomaster** » Thu Mar 18, 2021 2:45 pm

You can go up to those sizes ok. If you're looking for increased ride comfort, you can ditch the runflats if you haven't already

dhobbs · Post by **dhobbs** » Thu Mar 18, 2021 3:11 pm

Where does the z4 calculate it's speedo from? Is it the front or rear wheels?

sars · Post by **sars** » Thu Mar 18, 2021 4:26 pm

dhobbs wrote: ↑Thu Mar 18, 2021 3:11 pm Where does the z4 calculate it's speedo from? Is it the front or rear wheels?

From the gearbox output shaft

As for the OP, instead of increasing the tyre profile, choose a tyre that prioritises comfort over performance from a premium manufacturer and of course ditch the run flats. Something like Goodyear efficient grip

Mr Tidy · Post by **Mr Tidy** » Thu Mar 18, 2021 8:57 pm

sars has a point there, but you could do that and up the sidewall profile as well if it's comfort you are looking for.

I can't see you having any issues with rubbing because M models have 45 profile fronts and 40 profile rears.

mmm-five · Post by **mmm-five** » Thu Mar 18, 2021 9:48 pm

Mr Tidy wrote: ↑Thu Mar 18, 2021 8:57 pm sars has a point there, but you could do that and up the sidewall profile as well if it's comfort you are looking for.

I can't see you having any issues with rubbing because M models have 45 profile fronts and 40 profile rears.

Doesn't the M have slightly different offsets, and slightly higher ride height?

Mr Tidy · Post by **Mr Tidy** » Thu Mar 18, 2021 10:15 pm

mmm-five wrote: ↑Thu Mar 18, 2021 9:48 pm Doesn't the M have slightly different offsets, and slightly higher ride height?

I know the offsets are different, but I've seen people have fitted 224s to non-M cars without any issues.

I don't know about the ride height to be fair, but there are plenty of lowered Ms on Eibachs, H & Rs and coil-overs and/or with CSLs or Reps so I would have thought there is some space to spare.

z4newbie2021 · Post by **z4newbie2021** » Fri Mar 19, 2021 1:25 pm

I just had a look and i have a runflat on the back an xl on the otherside rear and 2 assorted Goodyears on the front i think i am going to fit MICHELIN PILOT'S all round it might feel less twitchy lol four different tyres cannot be ideal Thank you for all the reply's the forum is great

enuff_zed · Post by **enuff_zed** » Fri Mar 19, 2021 1:31 pm

z4newbie2021 wrote: ↑Fri Mar 19, 2021 1:25 pm I just had a look and i have a runflat on the back an xl on the otherside rear and 2 assorted Goodyears on the front i think i am going to fit MICHELIN PILOT'S all round it might feel less twitchy lol four different tyres cannot be ideal Thank you for all the reply's the forum is great

Wow! Yes that is most definitely the first step to take!

z4newbie2021 · Post by **z4newbie2021** » Fri Mar 19, 2021 2:40 pm

dhobbs wrote: ↑Thu Mar 18, 2021 3:11 pm Where does the z4 calculate it's speedo from? Is it the front or rear wheels?

Hi,
I think its fed from the differential so from the back.
cheers

dhobbs · Post by **dhobbs** » Fri Mar 19, 2021 5:12 pm

z4newbie2021 wrote: ↑Fri Mar 19, 2021 2:40 pm
dhobbs wrote: ↑Thu Mar 18, 2021 3:11 pm Where does the z4 calculate it's speedo from? Is it the front or rear wheels?
Hi,
I think its fed from the differential so from the back.
cheers

So the speedo will be calculated by the circumstance of the rear wheels, but fed through via the gearbox /diff.
I'm tempted to change my rear tyre sizes when they are due replacement. Seems the ///M sizes are slightly cheaper.
Daz

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