[475f30n55]

i have f30 335 xdrive.
i have done some researches and i have found out that i need to stay within 1% of rolling diameter difference...
what exactly happens if you go beyond 1%?
and im trying to put 245 35 20 front 255 35 20 rear.
when i put thoes numbers into calculator, rear is 0.7%(0.2") bigger in diameter and -7 in revs/miles(F 754, R747).
does revs/mile have to be same on xdrive cars??
i read too much things on google and completely lost. can somebody clarify it for me?

[alohasurftoad]

with the exception of f34, all f3x x-drives come in either a 225 wide square or 225/255 staggered setups with tire diameters of approx 26".

it is more complicated than this explanation but to keep things simple...

x-drive senses tire rotational speed, steering angle, gas pedal position and probably many other variables. individual tire speeds change when there are traction issues, such as slippery surface, drifting, etc. x-drive redistributes the torque going to each wheel to basically maintain the best traction.

if you put largely different diameters on an x-drive, at a given speed the small diameters would be spinning much faster larger diameters. the transfer case would "see" this difference in rotational wheel speeds as a difference in traction and it would be redistribute torque although there is no reason to, causing drivability issues and maybe transfer case malfunction.

although i've seen some on these forums with x-drive that have staggered diameter differences at 1.2%, most here agree 1% is max allowable and the closer they are matched is better.

regarding the 245/35R20, 255/35R20 setup you are inquiring about, the 0.7% diameter difference is okay for x-drive but the profile is too tall at 26.8" and 27". 245/30R20, 255/30R20 would be more appropriate. 25.8" and 26", 0.8% difference(still okay for x-drive)

[475f30n55]

Originally Posted by alohasurftoad

with the exception of f34, all f3x x-drives come in either a 225 wide square or 225/255 staggered setups with tire diameters of approx 26".

it is more complicated than this explanation but to keep things simple...

x-drive senses tire rotational speed, steering angle, gas pedal position and probably many other variables. individual tire speeds change when there are traction issues, such as slippery surface, drifting, etc. x-drive redistributes the torque going to each wheel to basically maintain the best traction.

if you put largely different diameters on an x-drive, at a given speed the small diameters would be spinning much faster larger diameters. the transfer case would "see" this difference in rotational wheel speeds as a difference in traction and it would be redistribute torque although there is no reason to, causing drivability issues and maybe transfer case malfunction.

although i've seen some on these forums with x-drive that have staggered diameter differences at 1.2%, most here agree 1% is max allowable and the closer they are matched is better.

regarding the 245/35R20, 255/35R20 setup you are inquiring about, the 0.7% diameter difference is okay for x-drive but the profile is too tall at 26.8" and 27". 245/30R20, 255/30R20 would be more appropriate. 25.8" and 26", 0.8% difference(still okay for x-drive)

so if the profile is too tall, does it means its going rub inside of fender?
btw thank you so much for informative explanatory.

[Er3k]

We need a post of everyone that has used a staggered non-runflat setups and post all of their specs, duration of mileage and brand of tires that worked well for them.

[475f30n55]

Originally Posted by Er3k

We need a post of everyone that has used a staggered non-runflat setups and post all of their specs, duration of mileage and brand of tires that worked well for them.

that sounds really good. also put on sticky, too

[shiftdnb]

Originally Posted by 475f30n55

so if the profile is too tall, does it means its going rub inside of fender?
btw thank you so much for informative explanatory.

The recommendation of change in total diameter size from stock to new is no larger than 3%.

[Er3k]

Originally Posted by 475f30n55

that sounds really good. also put on sticky, too

The F30 xdrive system seems to be more sensitive than when I had my E90. I really don't want to pay $1200 for OEM runflat tires when their are NONrunflats that could work just a well for half the price if we all knew the proper brands and specs.