[S6F]

I have the spare tire kit in my ‘19 540ixMsport. Thinking of moving to slightly larger diameter tires (235/45/19 instead of 245/40/19oem). Doing this strictly for comfort on lousy road surfaces. But the spare will now be 3% smaller. If I need to use the spare will this screw up the 4-wheel drive system? Appreciate advice.

[Superjackson]

Yes it will screw it up

[SteveinArizona]

indubitably!

[Chasta]

Originally Posted by S6F

I have the spare tire kit in my ‘19 540ixMsport. Thinking of moving to slightly larger diameter tires (235/45/19 instead of 245/40/19oem). Doing this strictly for comfort on lousy road surfaces. But the spare will now be 3% smaller. If I need to use the spare will this screw up the 4-wheel drive system? Appreciate advice.

Just ditch the spare completely for weight savings and make sure you have AAA or an equivalent. Lol.

[Knifeedge2k1]

Originally Posted by Chasta

Just ditch the spare completely for weight savings and make sure you have AAA or an equivalent. Lol.

This is the correct thing to do

[NTTY]

The DSC has means to calculate the actual diameter of each wheel and can adapt the x-drive locking amount of the multidisc clutch to a lesser extend to prevent distorsion in the drive train. The x-drive is safe in that respect.
The DSC uses the actual speed of each wheel to determine wheel slip. Would 3% difference confuse the DSC? Unfortunately this is not documented, but it's preferable to assume so.
I think driving temporarily with that difference is not an issue, to reach the closest dealer and get your tire replaced. But if the DSC throws any message at the dashboard, then stop driving and call assistance.
Generally, I would not recommend to change the diameter to that extend. 1% or so should be your max.
And note that there are more differences in comfort between two tire brands/models than in the profile (if you keep the same tire). Runflat tires are also uncomfortable whatever the profile. You can search the web to find the most confortable tire, and keep within the 1% diameter difference. Increasing the width by one size will keep you in 1% and will also slightly increase the profile (since it is a percentage of the width) and therefore the comfort.