Definition: The unit kilometers per hour (symbol: km/h) is a unit of speed expressing the number of kilometers traveled in one hour
Current use: Km/h is currently the most widely used speed measurement unit in the world and is frequently seen on speedometers and road signs. On speedometers in cars, both miles per hour and kilometers per hour are frequently displayed. The unit of kilometers per hour has several abbreviations (kph, kmph, k.p.h, KMph, etc.), but "km/h" is the SI unit symbol.

Miles per hour (abbreviated mph) is a unit of measurement for speed in the imperial and American customary systems. It represents the distance traveled in statute miles in an hour. A mile per hour (mph) is equivalent to 1.609344 kilometers per hour (km/h).
Current usage: When referring to the speeds of road traffic, mph and km/h are most frequently employed. The United States, the United Kingdom, and their neighboring countries and territories use it the most frequently. Although the Canadian road networks employ km/h, it is also used in the rail system there.

How to Convert Kilometers Per Hour to Miles Per Hour( Km/h to Mph)
The fastest way to convert kilometers per hour to miles per hour is to use this simple formula:
mph = km/h ÷ 1.609344.
The speed in miles per hour is equal to the speed in kilometers per hour divided by 1.609344. Since one kilometer per hour is equal to 1.609344 miles per hour, that's the conversion ratio used in the formula or use online calculator :