Where speed conversions actually come up
Driving in the US after living in Europe (or the reverse) is the obvious case — 60 mph = 96.6 km/h, 100 km/h = 62.1 mph. Marine and aviation speed comes in knots (1 kn = 1.852 km/h = 1.151 mph), used in weather forecasts and sailing manuals. Running / cycling apps often log one system while you plan routes in another; wind-speed reporting in weather apps is one of the last holdouts of mph in otherwise-metric countries.