If you're pulled over and the officer asks you how fast you were going, don’t say, “Why sir, I was traveling just under 70 kilometers per hour!” He might give you a quizzical look and ask you to step out of the vehicle. In the U.S., we measure the speed of our vehicles in miles per hour. So why then do our speedometers show not only miles per hour, but also kilometers per hour?
Most of the world uses kilometers per hour to gauge a vehicle's speed. The United States and the United Kingdom are rare exceptions (although since the UK uses the metric system, it makes their use of mph even more unusual).
With km/h there’s a wider range of speed limits that can be chosen. Kilometers are a smaller unit of measurement than the mile, meaning that speed limits can be dialed in more precisely than a larger, more encompassing unit of measurement.
Speed limit signs in the U.S. show the maximum speed that you can legally travel at, in terms of miles per hour. In fact, if you are a driver in the United States, you may not even know how to comprehend kilometers per hour. Ask yourself this: is 40 kilometers per hour fast or slow?
40 kilometers per hour is a relatively slow speed, translating to just under 25 miles per hour. The point is, despite having the measurements for km/h on our speedometers, it’s not really important to driving in the U.S. An unobservant driver might not even notice the secondary units of measurement on the speedometer.
In the 1980s the National Highway Traffic Safety Administration required that mph and or km/h be displayed on speedometers, but in 2000 it was clarified that speedometers must have mph, or mph and km/h. So it’s not actually required for speedometers to display km/h in the U.S.
Still, many vehicle manufacturers, including Subaru choose to display both. While not necessary in the states, a km/h reading can be quite helpful when you’re travelling outside the country. Both Canada and Mexico use km/h for their speed limit signs.
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