Quote:
Originally Posted by oldskewel
This post + thread might be helpful:
https://xoutpost.com/bmw-sav-forums/...ml#post1173532
In there, BMW says max of 2mm difference in tread depth (they appear to be assuming that the specified tire sizes are used, so nominally diameters are almost exactly equal).
With your difference of 275 vs 285, at an aspect ratio of 45, that is 4.5 mm smaller radius on the 275 tires. I have the stock 19" wheels, and once did the calculations to confirm that front, rear, and even the 17" spare all have almost identical diameters.
So you do seem to be out of spec, as you knew. About 9mm smaller diameter on a ~738mm diameter tire.
On my Cayenne, with 22" wheels, once when I had a flat and the collapsible spare tire on, I marked the tires and counted revolutions as my son drove it down the street in front of my house. Marked with chalk and masking tape. Very effective way to compare the actual rolling diameters vs. trying to measure with a tape measure or doing the math on tire specs. |
If 2mm tread depth is the limit according to BMW, then OP is slightly out of that spec. OP has about 3 mm difference if both tires are new, and has more difference if the small tires are worn out more. So it's possible that OP does indeed have a problem. I personally would think it would be OK, because other manufacturers allow for more % tread difference, and they are using the same kind of differentials. But I recognize I am not the expert.