Tire sizing tolerance

[one_bad_rover]

Hey all. Great space here. Ive had bimmers for quite a while but we finally pulled the trigger on a little 4.6is. The truck was 8K and we are into it for another 10k in repairs (stem guides, timing chain fun, the list goes on) But its at 100% currently.
Heres the dilemma, its an early 03. I bumped tire sizing pretty significantly, putting some 285/50/20s on the back and 265/50 20s on the front, my variance in revolutions per mile is 17. (646/663)
Everything seems fine, no crazy noises or lights, but after reading around here. Im a bit concerned. I dont see alot of people running larger tires, nor working these little guys much. This is a sport mobile for my wife. Its purpose is to get her bikes and paddleboards where they need to be.
Hence the beefier, more aggressive tires, she will be very light off-road but off road nonetheless.
Input, suggestions or things to keep an eye for? Thanks in advance!

[petebmwm]

You're fine, a few of us have bigger tires. No ill effects.

[tmv]

You're running taller but narrower tires. I wonder how that look (on stock style 87 I assumed)
It should be fine. The odometer will be a little off.
I would look into a set of 18" (not sure if fit) or 19" wheels which will be better for off-road purpose.

BTW, nice stable.

[oldskewel]

As I think the OP knows, the concern is the mismatch in tire diameter from front to rear. That will cause the transfer case to do more than it would if tire diameters were equal, as the truck was designed.

My understanding is that yes, 17 per mile will cause problems. But I can't find the source where I found that. I believe it should be no more than around 5. It could be that the mismatch would mainly be a concern for longer trips at highway speed, and will not be a problem for off-road kayak delivery.

[motordavid]

^ +1...I slightly recall a post/thread with good opins on tire rolling circumference differential, but can't find it.

That 2.5% wouldn't matter on a FWD or a RWD car, but it may impact the fixed F/R ratio of that older X drive...esp on highway for distance.
GL, mD

[Helihover]

I thought 2% was the max recommended. There's no lights on so its not too crazy. If it were me I would size them up closer or just go get a set of 18's. Oh wait I have a set, we can trade if you like

[JWMich]

The sizes you're talking about aren't a problem. I've run 275/60-18 square for quite a while, with zero issues. Some others have, too, in the past. Your 285s are basically the same thing.

The front to rear differential is your risk issue. I haven't seen anyone talk about running a diameter differential like you are. I'd suggest you pick a size and go the same at all four corners.

[Qsilver7]

Originally Posted by oldskewel

... the concern is the mismatch in tire diameter from front to rear. That will cause the transfer case to do more than it would if tire diameters were equal, as the truck was designed.
... I can't find the source where I found that. I believe it should be no more than around 5...

As other's have mentioned...the early e53 X5s with the permanent 32/68 split torque...uses ABS/ASC+T/DSC (aka DSC-X) to facilitate part of its AWD functionality. The info below says that a critical slip ratio greater than 5% will cause traction control regulation to occur (see info under "traction control system overview {ASC)"}. This can be in the form of engine throttle reduction or the application of ABS braking:

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[jdstrickland]

I do not know the answer to the question, but the issue is the difference from front to rear in the tires sizes he has selected.

The car does not care what tires are used, it only cares that they are all the same circumference. Any comments of a square fitment does not apply to the question asked.

The specific issue here is what effects the t-case will suffer when the front and rear are going different distances for long periods of time. The front and rear will always go different distances when going around a corner, but the difference is a transient conditin that will end as soon as the car is going in a straight line again. The difference in tire speeds over a long distance is the question, how different can the speeds be without killing the t-case?

The OP has already calculated the rotations per mile of his selected sizes, and he arrived at a difference of 17 rotations between front and rear. The differentials deal with the different speed/diameter/circumference from side to side, the t-case handles the differences from front to rear. He's asking about the potential harm to the t-case. On a square fitment, there is no harm. He's got a staggered fitment.

[jdstrickland]

Originally Posted by Qsilver7

As other's have mentioned...the early e53 X5s with the permanent 32/68 split torque...uses ABS/ASC+T/DSC (aka DSC-X) to facilitate part of its AWD functionality. The info below says that a critical slip ratio greater than 5% will cause traction control regulation to occur (see info under "traction control system overview {ASC)"}. This can be in the form of engine throttle reduction or the application of ABS braking:

This.

The deleted (by me) image goes on to say that the DSC will be activated when the speed difference is 2 mph, or greater. I'm not sure what this means in terms of tire rotations per mile, but the OP should be pretty close to getting his DSC to activate.