If I'm writing in US English, am I not allowed to use the metric system?

Question

For example, can I say this if my book is written in US English (in non-dialog):> The car was going at least 140 kilometers per hour!Or should I convert them to miles or what have you?It's for a fiction novel that takes place in Italy. However, I'm using it simply just to convey to the reader "the car was going super fast" in a fancier way.I'm self-publishing the book for the US/International market.

Answer 1

 In my view using kilometers is appropriate in a fictional novel set in Italy. Using the measurement references of the local venue could aid the author’s effort to promote the reader’s awareness of and interest in the place where the events occur. Such usage may help retain the reader’s attention by making the context appear more real. 

Answer 2

If you are writing in US English, using metric measurements will often lose your readers. Most Americans, for example, are not familiar with kilometers, and their relationship to miles; using Km/hour is lost on American readers, but would be easily accepted in England. In your case, use of foreign terms should be used only rarely. Describe the speeding car some other way, like it was moving at "breakneck speed", or the car was traveling at "lightning speed". By not using foreign terms, you avoid confusing the reader, and give yourself a clear road ahead for using creative, US wording.