The Metric System
Went to my first performance rally in North Bay, ON, Canada in '92.... we were still using a mechanical ODO for the co-driver in those days. No way we would have the 1.6 difference in calibration gear ratios to make the mechanical ODO read properly.
So, imagine getting there, being handed the routebook, and having to furiously convert everything from kM to miles less than an hour from the start!