[Pro55]

So my manual says there should be no Significant difference between tread on both front and rear axles.

Its a bit vague. What one person calls significant another doesnt. If the X-drive is so critical surely there would be a mandated limit.
Otherwise all repairs under warranty and costing a fortune for bmw.
The fact no limit is mandated leads me to think the 1% difference in circumference I’ve seen in other threads is probably urban myth!! And applied to early x-drive vehicles.
Anyone have any hard concrete experience with this?
What difference in mm can be accepted between axles/tyres.?

[Ninefourteener]

On manual transmission cars, with no slip between F/R axles..... maybe.... particularly on older cars. This was a big deal on some of the early WRXs, STIs and EVOs.

However..... with newer vehicles, with programmed slip, automatic transmissions, etc.... nothing to worry about.

If one tire was 10psi lower in pressure than the other (which is the equivalent of what you're asking)..... You're not in any danger of damaging a differential.

There's a video on here somewhere of a guy doing burnouts/donuts in his M50i. Rear tires are smoking.... front tires are barely moving. Our cars are rear-biased, and designed to slip.

I would advise getting the exact same brand/model/size tire.... because there are often differences between brand's measurements..... but if you need 1 tire, just buy 1 tire.