This app was mentioned in 5 comments, with an average of 3.40 upvotes
I think/heard the reason they don't is so people can't blame (read: take legal action against) Google if they get a speed ticket but Maps says they were going under the limit.
Either way, I use this and it works really well:
https://play.google.com/store/apps/details?id=com.amphebia.navigationspeedometer
I've used this app for a year or two and it's been great:
https://play.google.com/store/apps/details?id=com.amphebia.navigationspeedometer&hl=en
It's paid, but I used free rewards credit to purchase it.
Plus when I use it, it looks like it's part of google maps and meant to be there.
I picked up Maps Speedometer a while back. I did so mainly for a trip from Seattle to to Salt Lake City and back. I kept it on because it's actually pretty handy. I don't do the alerts for over the limit now but that worked well for the main trip. It's nice to be able to reduce my glancing away from the road to the speedometer whenever I need navigation as well. (Common for me since I work at client locations which can change.)
Yo! I think I've accidentally stumbled on what you need.
https://play.google.com/store/apps/details?id=com.amphebia.navigationspeedometer
I had to get this, since my gf didn't really liked the idea of the following the traffic, while driving my car, where the speedometer isn't exactly working.
It works as an overlay on top of whatever app you are using (e.g Maps).
And of course it displays glorious KM/H