Road, sea and air use different units for a reason
Road speeds use mph or km/h. Shipping and aviation use knots — one nautical mile per hour — because a nautical mile is one minute of latitude, which makes navigation arithmetic on a chart straightforward. Physics uses metres per second because it is the SI unit and drops cleanly into equations for acceleration and energy.
The cleanest of these is metres per second to kilometres per hour: multiply by exactly 3.6. There are 3600 seconds in an hour and 1000 metres in a kilometre, so the factor is 3600/1000 with no rounding at all.
Speed limits and stopping distances
Common limits convert to awkward numbers, which is why signs are never direct translations: 30 mph is 48.3 km/h, 60 mph is 96.6 km/h, and 70 mph is 112.7 km/h. Countries round to their own convenient figures rather than converting exactly, so a 50 km/h urban limit is 31 mph rather than 30.
One thing conversion cannot smooth over: stopping distance grows with the square of speed. A car at 60 mph needs roughly four times the braking distance of the same car at 30 mph, not twice.