Quote:
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According to the experts at TireRack, the tires on xdrive cars can be different sizes from original equipment (but must be within 3% of original tire diameter), and they need to be within 1% (not to exceed) diameter size of each other to not cause problems.
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Some quick math:
31/2*0.01=0.155
Just about 5/32" difference in tread depth when starting with identical tires.
https://www.carbibles.com/calculators/
I use this site for comparison of sizes (to choose a spare tire size for example, or determine what size to use when picking a wider tire and keep speedometer "accurate". (In quotes because BMW adds 3mph to actual speed other than in Japan)
Looking at the two main sizes spec'd for E53: 255/55/18 and 235/65/17 and they are only 1.2mm difference in rolling circumference or 0.05% difference.
(Stunningly similar sizes).
I have pre xdrive which can handle more difference with open differential split using a 2/3 ratio planetary gear in the transfer case and my spare is about 1.5% difference in rolling circumference.
I would measure the actual tire circumference with a tape measure (wrap with the numbers toward the tires) not knowing how the mfg tolerance will affect things.
2225/25.4/100*8=7.008
7/8" difference in circumference. (a number large enough to get an accurate measurement.
To get an accurate rolling circumference, you could mark the tire with some chalk and drive, measure between the marks on the ground.