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.