G29 M40i winter wheel options

G

Glaikit

Member
Have an M40i and want to avoid 799s due to cost and rarity of second hand sets for use as a winter setup. Am considering investing in these as the car can be skittish on wet/cold surfaces and have an occasional 80 mile commute on country A roads.

Has been said that an 18 inch winter setup up would fit the M40i but was dubious about M performance brake clearance. Was considering taking a punt on 19 inch 791M wheels as they are readily available second hand and ChatGPT reckons they should work. Also gives more tyre options. Unsure about how they would sit in the arches though! Anyone got these on their M40i?

Otherwise, does anyone have recommendations on wheels for the M40i outside of the 799s that won’t break the bank?

Thanks in advance.
 
What size brakes are on the M40i?

I have the M-Performance 370mm Brembo set-up on my 335xD, and I've just put on my winter set of 18's.

As long as the offset & width is correct then most genuine BMW 18" options (or aftermarket ones of the same dimensions) should fit. You may even get away with 17's if the brakes are smaller.

For example
 
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mmm-five said:
What size brakes are on the M40i?

I have the M-Performance 370mm Brembo set-up on my 335xD, and I've just put on my winter set of 18's.

As long as the offset & width is correct then most genuine BMW 18" options (or aftermarket ones of the same dimensions) should fit. You may even get away with 17's if the brakes are smaller.

For example
Click to expand...
Thanks for your input, really appreciated. As far as I know the fronts are 348mm and rears are 325mm so might get away with 18 inch rims as you say.

Appreciate you posting those eBay links, the set of 798s might be a good option by the looks of it.

According to ChatGPT the offset is different on the 791 so would be a bit of a risk?:

  • The offset (ET) matters for brake & caliper clearance. The Z4’s OE ET32 front is shallower than the ET27 of one 791M listing (so wheel sticks out more) but using an 8J width with ET27 may push the inner lip closer to the suspension components/brake calipers.
  • I didn’t find a definitive statement that 791M has been tested on the Z4 G29 M40i with the big M-Sport brakes and that clearance is guaranteed. The OE wheel for the Z4 (for example the 799M style) is designed for the M40i’s braking system. So using a narrower & different offset wheel introduces risk of interference (calipers, suspension, inner arch).
 
You've got to be careful when reading AI results or ensuring you've input the correct info into the initial prompt...in the case above it's not even comparing the 799 to the 798 - it's comparing a 791!

Yes, the offset is important, but you also have to take into account the width, as a narrower wheel (say a 8j vs 8.5j) will already have a little less tuck (i.e. the tightness to the suspension) of about 1/4" or 6mm...so a lower ET on a narrow wheel will mean the same or larger overall gap to the suspension and wing.

You can use something like willtheyfit.com to check the movement of the wheel in respect to the suspension/wing.

In your case, your front is a 19" 799M 10x19 et40 with 255/35r19 tyre and the 798M is 9x18 et32 with 255/40r18 tyre (according to RealOEM specs for your model):

So that 1"/25mm narrower wheel means you get 12.5mm extra space on each side of the wheel - but the 8mm smaller offset means the inside will move 20.5mm (i.e. 12.5mm+8mm) out and the other 4.5 (12.5mm-8mm) is taken away from the outer face so the outer face is a little more tucked into the arch
www.willtheyfit.com

Alloy Wheel & Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit

Alloy Wheel & tyre fitment calculator - it calculates inset, poke, speedo error, tyre stretch and loads more.
www.willtheyfit.com www.willtheyfit.com

2025-11-15_14-23-07.png
 
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mmm-five said:
You've got to be careful when reading AI results or ensuring you've input the correct info into the initial prompt...in the case above it's not even comparing the 799 to the 798 - it's comparing a 791!

Yes, the offset is important, but you also have to take into account the width, as a narrower wheel (say a 8j vs 8.5j) will already have a little less tuck (i.e. the tightness to the suspension) of about 1/4" or 6mm...so a lower ET on a narrow wheel will mean the same or larger overall gap to the suspension and wing.

You can use something like willtheyfit.com to check the movement of the wheel in respect to the suspension/wing.

In your case, your front is a 19" 799M 10x19 et40 with 255/35r19 tyre and the 798M is 9x18 et32 with 255/40r18 tyre (according to RealOEM specs for your model):

So that 1"/25mm narrower wheel means you get 12.5mm extra space on each side of the wheel - but the 8mm smaller offset means the inside will move 20.5mm (i.e. 12.5mm+8mm) out and the other 4.5 (12.5mm-8mm) is taken away from the outer face so the outer face is a little more tucked into the arch
www.willtheyfit.com

Alloy Wheel & Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit

Alloy Wheel & tyre fitment calculator - it calculates inset, poke, speedo error, tyre stretch and loads more.
www.willtheyfit.com www.willtheyfit.com

View attachment 287146
Click to expand...
Thanks for the full breakdown, yes I originally was looking to see if a set of 19 inch 791 would work given they are more common.

Think I’ve decided to track down a set of use 798M as they are the recommended size from BMW 👍🏻
 
Glaikit said:
Thanks for the full breakdown, yes I originally was looking to see if a set of 19 inch 791 would work given they are more common.

Think I’ve decided to track down a set of use 798M as they are the recommended size from BMW 👍🏻
Click to expand...
The 798M also has the benefit of being 18", so you get a more compliant ride over speedbumps and rough winter roads...and the winter/all-season tyres in 18" are a bit cheaper.

Looking at the specs of the 791M wheels, they're narrower again, so would easily clear the suspension & sit under the arches nicely...and as a 19" wheel they'd easily clear the brakes...although I'd prefer wider wheels as they're quite a narrow 8J front, meant for a 225/235 tyre.
 
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