Definitely KPH since actually driving at 200MPH would have the background whizzing past you much faster. Here's a vid, timestamped to when a driver reached 200mph.
Good, now I wish there was a mod that changes lighting settings when you're inside a vehicle. Shadows don't properly render when you're in some vehicles. I don't know why.
They usually have blueprints that let you craft them to your level. But there's one smart weapon that you can find that automatically scales to your level no matter when you use it.