As a lifetime reader and lifetime US resident, the phrase "The car was going at least 140 kilometers per hour!" wouldn't mean a lot to me. I know enough to know miles are pretty different than kilometers, but I'd have to look it up to know if that's actually impressive. When I've traveled to Canada (which primarily uses KM/H), I've generally just tried to keep the numbers on the speed gauge near the numbers on the road signs; I don't really grok how fast I'm travelling at those numbers at all. I'm guessing it would take more than a few weeks of driving there for it to sink in.
The important thing here is whose voice was that phrase written in? The hint to readers there is now they know that isn't how a US resident would think.
If the narrator is Italian, that's great. They aren't a US resident, and its probably good to establish that. I now know as a reader that this is a non-US person thinking in non-US terms. I probably need that. The fact that they found that speed really impressive is really all I need to know.
If they are a US tourist though, then I just saw through to the (non-US) writer. In that case, you really need to rephrase this into something that would sound impressive to someone who, like your narrator, has little clue what 140KM/hr means.
After looking it up, and seeing it's about 87MPH, I still wouldn't be horribly impressed (without more context). I generally try to travel about 8-9 MPH over the limit (to avoid tickets), and on most of the highways here in Oklahoma the limit is 75. That means I regularly travel at 84MPH with no trouble from the law at all. There are lots of places in states NW of here where the limit is 80, so I'd be going near 90 if road conditions allow.
87 is a good clip, don't get me wrong, but likely not worthy of an exclamation mark if you are traveling on a highway.