[Score04]

Hello,

I was researching and couldn’t find details. Anyone know why X3m has 255/45/20 front and 265/45/20 wheels while m40i has 245/45/20 front and 275/35/20 wheels? I was considering m40i but prefer all four wheels to have 45 profile living in Chicago that would benefit from pothole capital. Would It be ok to put 45 profile on the m40i in the rear with the same 275 tires or will that screw up the variance from speedo a lot? Any advice would be great as well as explanation regarding these two models tires setup in oem form in 20s. Thanks!

[suby01]

Good questions subscribing.

[Score04]

Bump

[DoubleZeroNineOneEight]

What size are your rims?
Tire width (mm) x Sidewall ratio (%) = sidewall height.
275/45R20 = 754 diam.

[Score04]

Unfortunately, I'm not understanding what I should get out of the chart. I appreciate you populating it but if you can put a little note answering my original question that'd be great!

[PostModernHero]

Originally Posted by Score04

Hello,

I was researching and couldn’t find details. Anyone know why X3m has 255/45/20 front and 265/45/20 wheels while m40i has 245/45/20 front and 275/35/20 wheels? I was considering m40i but prefer all four wheels to have 45 profile living in Chicago that would benefit from pothole capital. Would It be ok to put 45 profile on the m40i in the rear with the same 275 tires or will that screw up the variance from speedo a lot? Any advice would be great as well as explanation regarding these two models tires setup in oem form in 20s. Thanks!

As it is a ratio, there is still plenty of sidewall on the 275 width tires on the 20 inch rims. You said 275/35/20... they are actually 275/40/20, which pretty much have equal sidewall to the fronts at 245/45/20. The 21 inch rims are 275/35/21, so that must be the source of your confusion. I wouldn't worry about it...

[Score04]

Originally Posted by PostModernHero

As it is a ratio, there is still plenty of sidewall on the 275 width tires on the 20 inch rims. You said 275/35/20... they are actually 275/40/20, which pretty much have equal sidewall to the fronts at 245/45/20. The 21 inch rims are 275/35/21, so that must be the source of your confusion. I wouldn't worry about it...

Yes, you are correct, the rears are 275/40/20. I was inaccurate. I guess I meant to ask would it be okay to put 275/45/20 and have 45 profile on all 4's. In this chart what is the most important thing to realize what I can and can't do as options? Thanks!

[PostModernHero]

Originally Posted by Score04

Yes, you are correct, the rears are 275/40/20. I was inaccurate. I guess I meant to ask would it be okay to put 275/45/20 and have 45 profile on all 4's. In this chart what is the most important thing to realize what I can and can't do as options? Thanks!

I'm not understanding why you are wanting to do that? 45 sidewall at 275 width will make the wheel diameter larger overall, and it will have a *taller* sidewall than the fronts. 40 sidewall at 275 will equal the sidewall height of the 45 sidewall of the fronts.

The X3M has 45 all around, because the tire widths are closer to being the same (255/265 vs 245/275).

What you are thinking would keep things equal with all four tires having the same width, but that isn't the case. On a square setup, it would be like having 45 profile tires on the front with 50 profile tires in the back...

[Yellowmattei]

I investigated this possibility when I bought my X3 last October, and after lots of conversations with quite a few people, it became clear that having a 45 aspect ratio on a tire, vs a 40 will not offer any more protection against potholes. I spoke to a BMW technician, the people at Tire Rack, and a tire store here in the New York area, where we suffer just as badly from potholes as you do in Chicago. Changing to the X3 M size won't help you at all that way. One thing that can help is switching to non run flat tires.

This is just something that comes with the territory of having lower profile tires.

Good luck.

[suby01]

Originally Posted by PostModernHero

I'm not understanding why you are wanting to do that? 45 sidewall at 275 width will make the wheel diameter larger overall, and it will have a *taller* sidewall than the fronts. 40 sidewall at 275 will equal the sidewall height of the 45 sidewall of the fronts.

What you are thinking would keep things equal with all four tires having the same width, but that isn't the case...

AAAAh so it seems. If the tire is wider but has the sidewall # lower it will still equal the same sidewall of a narrower tire.

[teaston]

Originally Posted by Score04

Hello,

I was researching and couldn't find details. Anyone know why X3m has 255/45/20 front and 265/45/20 wheels while m40i has 245/45/20 front and 275/35/20 wheels? I was considering m40i but prefer all four wheels to have 45 profile living in Chicago that would benefit from pothole capital. Would It be ok to put 45 profile on the m40i in the rear with the same 275 tires or will that screw up the variance from speedo a lot? Any advice would be great as well as explanation regarding these two models tires setup in oem form in 20s. Thanks!

The X3M comes only with 21" wheels, at least it does in UK.

If they really are available in those sizes in US, I guess they're a compromise made for the american market.

[Score04]

Originally Posted by PostModernHero

I'm not understanding why you are wanting to do that? 45 sidewall at 275 width will make the wheel diameter larger overall, and it will have a *taller* sidewall than the fronts. 40 sidewall at 275 will equal the sidewall height of the 45 sidewall of the fronts.

The X3M has 45 all around, because the tire widths are closer to being the same (255/265 vs 245/275).

What you are thinking would keep things equal with all four tires having the same width, but that isn't the case...

It was just that as I mentioned and someone else. 45 sidewall protection for pothole Chicago vs having 40 profile. I understand the match up due to width as is being explained. It was for simplicity of pothole protection.

Honestly, I would now prefer to have the x3M 20" wheel size than the m40i

[PostModernHero]

Originally Posted by Score04

It was just that as I mentioned and someone else. 45 sidewall protection for pothole Chicago vs having 40 profile. I understand the match up due to width as is being explained. It was for simplicity of pothole protection.

Honestly, I would now prefer to have the x3M 20" wheel size than the m40i

You're still not getting it. "Pothole protection" is in direct relation to sidewall height. 45 and 40 are equally tall with the stock staggered setup! You're being stuck on the number...

[dmicah]

If you're focused on having an aspect ratio of 45 all around, then I would just go with the square setup of 245/45-20. I kinda wish I had a square setup, TBH

I'm curious: you understand that 45 is a ratio rather than millimeters, right? That, I think, is what PostModernHero is trying to tell you. If you multiply the section width by the aspect ratio you get sidewall height:

275 x 40% =110 MM
245 x 45% = 110.25 MM

So they essentially have the same 110 MM of sidewall rubber to fight off Chicago potholes.

[Score04]

Okay.

[crst4s]

The general rule for all wheel drive vehicles is that dia (or circumferance) of wheel and tire should differ by no more than 1%. Hence 245/45/20, 275/40/20 M40i, 255/45/20, 265/45/20 X3M. The difference is likely in the interests of handing. Wider front tires give better front end grip which in turn allows for more rotation (i.e., less understeer or push). The all wheel drive system in the X3M is from the M5 and engineered for greater rear wheel bias than the M40i.

[spta97]

Originally Posted by teaston

The X3M comes only with 21" wheels, at least it does in UK.

If they really are available in those sizes in US, I guess they're a compromise made for the american market.

The US model comes in competition (21”) and non competition (20”).

I have the 21” comp but run 20” square for my winter tires.

OP - even with my 21” there is a fair amount of sidewall compared to my 19” E46 M3 Competition which wore basically rubber bands (and I bent a rim and split the sidewall on a pothole - I’m NY).

I learned it’s important to keep a high pressure as this will transmit the impact to the shock rather than the tire and wheel.

What’s ironic about this thread is most X3M guys in the forum are complaining that they should have wider tires than the M40i and several have upgraded to larger sizes.