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Source: https://writers.stackexchange.com/a/34761
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

Many have mentioned that you might use km/hr as a writing tool to create an effect. I'd like to offer a global answer: if you're not intentionally using km/hr to create an effect, use mph.

If you use a number in writing, the reader has to interpret that. To do so, they will use things like context clues. Some SI units are decently easy for people to interpret. If I say someone struggled to lift a 20 kg mass, we can put 2 and 2 together pretty easily and get an intuitive feel for how heavy that object is.

km/hr, however, is a unit that most Americans are not very familiar with at all. For most of us, it's just "the little numbers on the inside of the speedometer that we always ignore." Myself, I work with SI units all the time by trade, and I still don't have a decent feel for km/hr. If you gave me 140 km/hr, I would be obliged to do the mental juggling required to convert it to 87 mph, and then I could compare it to highway speeds and get a sense of how fast it is.

If you want to use km/hr, make sure you give the reader strong hints as to how fast that is. Make sure the verbiage includes something like a reference to flying along at speeds that would get you pulled over on many highways. Or maybe it gets compared to the speed of a major league fastball.

On the other hand, if your text has one of the characters mentioning it's 70 km to a particular city in Italy, knowing that they are traveling at 140 km/hr does tell the reader that they have a 30 minute drive ahead of them. It doesn't give a sense of how fast they are driving, but it does give a sense of how long the characters have to relax before they get there.

Original score: 7