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Source: https://writers.stackexchange.com/a/34858
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

If the majority of your readers are from US, then mph will be more easily interpreted. If part of the goal is to make people feel like they are in Italy, then metric would be better. Context could help readers, an accident on the freeway allowed bob to creep along at 4 kph, while Marry zipped through the school zone at 55 kph and had to pay a speeding ticket. The issue is both with the units, and with the laws. So the speed through a school zone is 25 mph in the USA, but that does not mean that it is 25 mph (40 kph) everywhere in the world.

Autobahn speed record 268.8 mph (432.59 kph) according to one web site: [https://auto.howstuffworks.com/5-fastest-speeds-on-autobahn10.htm](https://auto.howstuffworks.com/5-fastest-speeds-on-autobahn10.htm)

Really good tries are slightly over 200 mph.

Wikipedia has a good site documenting freeway speeds in the US that range from 60 to 85 mph (96 to 137 kph). You can bet that people in many of these areas are driving 5 to 10 mph above the posted limit. Thus, "fast" will depend a bit on where you are from.

[https://en.wikipedia.org/wiki/Speed\_limits\_in\_the\_United\_States](https://en.wikipedia.org/wiki/Speed_limits_in_the_United_States)

Original score: 1