Google Maps is not shy borrowing useful features from Waze, following the acquisition of the company by Google. Just recently Maps gained indication for speed cameras, speed traps and the option to report possible hazards on the road. Now, a speedometer has been implemented on the navigation screen to help you stay within speed limits.
The app warns you if you are exceeding the speed limit while the speedometer is displayed in the lower-left corner of the navigation screen. That's another feature coming straight from Waze but unfortunately, the speedometer rollout is staged and only a few users report being available.
At this point commuters in the US, Belgium, UK and Brazil can take advantage of the speedometer while the rest of the world will have to wait a little bit longer.
You can try looking for the speedometer in the Navigation Settings menu to check if it's available in your region.