I am sure it is some sort of a DOT requirement for US to have both.....
Some GM cars only have one reading and you can easily change miles to km's through the options menu and your selection lights up....(rental car discovery). BMW should of thought about that....
When you come down south you can easily change your digital speedometer to miles.
Current: 2016 F10 535i xDrive AW w/ Black Dakota, ZMM, ZCW, ZDA, ZDB, ZPP, 688 & Black Grills.
Retired: 2011 E92 M3 -AW
....ED April 29th, 2010
Retired: 2007 E92 335i -Space Gray.....ED Oct 19th, 2006