How can I turn on google maps speedometer alerts on exceeding the speed limits?
Both these apps appear to meet the requirements (not tested by me). I have requested OP to add/edit based on their tests.

You can try Velociraptor - Speed Limits &amp; Speedometer, which is free and links to Open Street Maps (OSM) as also appears as an overlay  on Google Maps (select auto Launch in advanced settings). It is free &amp; Open Source


Features:
• Automatically displays in any apps you choose (e.g. Google Maps)
• Intelligent caching &amp; fast refresh of speed limits
• Sound alert when speed limit is exceeded
• Speed limit tolerance: % and integer amounts
• Transparency, size, and tap to hide setting


Alternatively, app Maps speed limits . It's a paid app with one week free trial


Maps Speed Limits shows the current speed limit in Google Maps or any other app.
Speed limit alert: play a sound when speeding

Barlop / OP adds
Looking at the screenshot below, you see on the left of the screenshot. (red circle aside), Velociraptor showing speed limit and speed.
Also on the left, you see the red circle, that's map speed limits. (not so necessary as Velociraptor shows both speed and speed limit anyway).
The speed mentioned on the very bottom is google maps saying that, google maps suddenly decided to state the speed!
I don't mind having both, sometimes the speed limits reported can differ, as the speed limits use a different database and they can update at different positions on the road. Velociraptor makes clear it uses OSM, and i'm not sure what database map speed limits uses.
They work internationally and have an option to switch between kph and mph.
Velocirapter also has quite comprehensive settings too and is pretty easy to navigate. And the google play app velociraptor page shows some screenshots of the settings it has too.
It is an overlay so google maps is open there, and any app showing the speed or speed limit is showing it as a floating  overlay over it.

Here is another pic from here incase anybody doesn't believe that the above is google maps (it is). As is the pic below. And since its google maps it applies to works for whatever country you are in.