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Source: https://writers.stackexchange.com/a/34755
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

Whether you're allowed: yes, you're allowed to use metric in US English.

Whether it's a good idea depends very much on details of the setting, and of your intended audience.

If your story is set in the rural United States in, say, the 1950s, none of your characters are likely to think in metric units; even a scientist who regularly uses metric in his work would be accustomed to mentally translating, or doing his work in metric, but using US Customary for daily life. If your setting is a starship with an internationally derived crew, it might well be that only the elderly Americans among them would still think in pounds, miles, and Fahrenheit.

Then again, if your intended audience is a scientifically literate America, at least they'll know that 140 km/hr is well above normal highway speed (for America), they will recognize that "subzero" temperatures might only mean below freezing (as opposed to Arctic conditions), and they won't be confused to hear someone's body mass given in kilograms or height in centimeters.

Bottom line: as long as your usage is consistent with your setting, and your audience won't get lost (and you use the units correctly -- I recall a novel where a "roaring gale" was given as "above 35 km/hr"), you should be fine.

Original score: 60