I don't know on the i3 how it determines road speed, but it does use the wheel rotation sensors to check for uneven rotation rates to detect what it determines to be slippage. I do know that there's a computer in the loop, as BMW has chosen to increase the indicated speed by 2-mph across the range to account for slight differences in tires and to meet the German requirement that the speedometer can't read lower than your actual speed, but it appears that the odometer is accurate.
On most systems today, the speed sensor is now an electronic sensor in the drive-train, often the transmission, not at a wheel, since with the differential, and going around curves, the individual wheels' rotation will vary except if you're going perfectly straight, and the tires are all like new.
I did a quick search on a parts breakdown page (www.realoem.com
), but it was not obvious where the actual sensor is from there.