The factors
A mile per hour is exactly 0.44704 m/s, because a mile is 1,609.344 m and an hour 3,600 seconds. A kilometre per hour is 1/3.6 m/s, so multiplying m/s by 3.6 gives km/h: 10 m/s is 36 km/h. A knot is one nautical mile, 1,852 m, per hour, so 1 knot is exactly 1.852 km/h and about 1.151 mph.
Speed limits
Common limits in mph: 20 mph is 32.19 km/h, 30 mph is 48.28 km/h, 50 mph is 80.47 km/h and 70 mph is 112.65 km/h. The other way, 50 km/h is 31.07 mph and 100 km/h is 62.14 mph.
One pair at a time
The pairs people look up most have a page each, opened on that pair with a table of common values: km/h to mph.
Questions
Why do ships and planes use knots?
Charts measure distance in nautical miles, which match minutes of latitude, so a speed in knots works straight onto the chart. 30 knots is 34.52 mph or 55.56 km/h.
Can it turn a running pace into a speed?
No. A pace such as 5 minutes per kilometre is time per distance, the other way up from a speed, and is not in the list. Divide 60 by the minutes: 5 minutes per km is 12 km/h.