Here's how to add a speedometer to Google Maps on Android
Luckily, now, a clever work-around brings a speedometer to the Google Maps application. The way it works is via a third-party app - the Velociraptor Map Speed Limit - that displays a notification on top of the running Google Maps app, showing you the speed limitation for a particular area.
In the settings of this neat third-party app, you can adjust the units of measurement you want to use as well as the speed threshold that you will get warnings about. It's not the most elegant solution, but until Google figures out a native one, this one is as good as it gets.