[IanH]

Xdrive isn't full time awd, so why would it have an issue with 2 different diameters? It could just slip the clutch to compensate, and likely does this regularly for all sorts of driving conditions.

The only thing that can really be an unsolvable issue would be the computational elements based on wheel speed, which shouldnt cause xdrive to fail but it may not work to its fullest capabilities.

[mjr24]

Originally Posted by Hiddenknife

I know it might be stupid sounding question, but does your BMW have xDrive, do you run non star marked non runflat tires? For how long and if any issue so far?

BMW star marked tires are mostly "star marked" because they are approved in rolling circumference (same RC all four)but of course there are some other star marked tires that use different rubber compound like Michelin Pilot Sport 4S

I’ve put hundreds of thousands of miles on high end BMWs with zero issue…..ever. Maybe ran OEM tires for 10 percent of that…..yes lots of that were on XDrive models….X5M, X7, M5, M8, M550i.

It’s like how BMW recommends Shell gasoline. Sure, maybe the car runs “optimally” on it….but the gas from other places works just as good.

That being said, I stay within the 1 percent rule. That’s something I wouldn’t want to risk going outside of.

[Hiddenknife]

Originally Posted by mjr24

I’ve put hundreds of thousands of miles on high end BMWs with zero issue…..ever. Maybe ran OEM tires for 10 percent of that…..yes lots of that were on XDrive models….X5M, X7, M5, M8, M550i.

It’s like how BMW recommends Shell gasoline. Sure, maybe the car runs “optimally” on it….but the gas from other places works just as good.

That being said, I stay within the 1 percent rule. That’s something I wouldn’t want to risk going outside of.

If i strictly follow the tiresize, load rating, speed rating written on the B pillar, does that mean im going to be within the 1%?

[pbonsalb]

Yes. Same size tires as stock are what BMW specifies. When you alter sizes, stay very very close to the original front to rear circumference variance, especially on AWD cars. If that variance is 1%, don’t exceed 2%. The traction control, ABS and AWD transfer case are programmed for that stock variance.

[RockCrusher]

Originally Posted by smrtypants44

Yeah if BMW had such tight tolerances on xdrive then units would be failing all over the place just from manufacturing and wear variances.

The Porsche sanctioned tires identified with the 'N' on the sidewall have a bit less tread depth. I believe this arises because the tire maker removes some tread to ensure the tires sizes are more precise/matched that what the result from the tires being used directly from the mold.

The BMW sanctioned tires I looked into likewise have less tread depth.

[RockCrusher]

Originally Posted by IanH

Xdrive isn't full time awd, so why would it have an issue with 2 different diameters? It could just slip the clutch to compensate, and likely does this regularly for all sorts of driving conditions.

The only thing that can really be an unsolvable issue would be the computational elements based on wheel speed, which shouldnt cause xdrive to fail but it may not work to its fullest capabilities.

xDrive is a full time AWD system.

BMW xDRIVE technology is an advanced, permanent all-wheel drive system that ensures your BMW has the best possible traction at all times. This helps to keep you safe in inclement weather and in imperfect driving conditions, and helps provide smooth, sporty performance.

https://www.bmwofsudbury.com/new-bmw...ippery%20roads.

The system can adapt to some differences in tire diameter. But slipping the clutches can (will) lead to premature wear. Which is one reason why the proper tires should be used from the get go.

[IanH]

Originally Posted by RockCrusher

xDrive is a full time AWD system.

BMW xDRIVE technology is an advanced, permanent all-wheel drive system that ensures your BMW has the best possible traction at all times. This helps to keep you safe in inclement weather and in imperfect driving conditions, and helps provide smooth, sporty performance.

https://www.bmwofsudbury.com/new-bmw...ippery%20roads.

The system can adapt to some differences in tire diameter. But slipping the clutches can (will) lead to premature wear. Which is one reason why the proper tires should be used from the get go.

That description is wrong, the front wheels can be completely decoupled to 0 torque, aka clutch release. The rear wheels are driven at all times.

here is a good writeup

https://f30.bimmerpost.com/forums/sh...38&postcount=2

[visionaut]

Tread depth 2mm between tires, is 4mm diameter difference..

I run the Michelin PS4S 245-255-35/19 combo on my F36 435iX. Michelin’s exact difference (all brands/styles aren’t same-same) is only 8 revs/mile. So even tho’ the diameter is cited as .2” (25.8 “ vs 26”) that’s probably rounded up.

My OE staggered 19s (225/40 & 255/35) were 1.5mm diameter difference.

It’s my understanding that BMW’s tire inflation specs for specific tire sizes are how they make up the small differences to try to get equal F & R. They’ll slightly over or under inflate the front or rear axle to match up… (They raised rear pressure, the 255 being smaller than the 225. I run my 245 fronts slightly higher since they’re the shorter.)

Regarding BMW ‘Star’ tires. They’re different than the non-marked versions. They’re developed by the car & tire manufacturer in combination, tweaking the ‘standard tire’ to the car makers priorities. Good write-up here:

https://www.michelinman.com/auto/aut...d-marked-tires