The factor is the mile itself
Speed is distance over time, and the hour is the same on both sides, so mph to km/h is just miles to kilometres: multiply by 1.609344, exactly. 60 mph is 96.56 km/h, 100 km/h is 62.14 mph. The US, UK and a handful of other countries post limits in mph; nearly everywhere else uses km/h, and a car imported across that line reads wrong on its speedometer by a factor of 1.6.
kilometres per hour = miles per hour × 1.609344 | miles per hour = kilometres per hour ÷ 1.609344
Rough conversion in your head
Add 60% to mph: 50 mph → 80 km/h (true 80.5). Multiply km/h by 0.6 the other way: 130 km/h → 78 mph (true 80.8). The classic pairs are 30 mph ≈ 50 km/h, 50 mph ≈ 80 km/h and 70 mph ≈ 113 km/h. For m/s, divide km/h by 3.6: 100 km/h is 27.8 m/s.