Not positive of anything, but, the difference in OD is about 1.4%, which, at say 30mph, may not be enough to show up. The 2mph buffer is a constant above the design speed, however that's detected. But, at 30mph, 2mph is 6.67%, so 1.4 isn't that big of a difference (less than 0.5mph). The vehicle is not using GPS for determining speed or odometer, though. That leaves the gear ratio and the tire size. I'm sure it's not a mechanically driven cable, but the detector must get its reference from some rotating device, and changing the OD of the tire/wheel assembly will throw off that actual correlation.
2011 535i x-drive GT, 2014 i3 BEV