If you have a mount for your phone, there are plenty of excellent standalone GPS speedometers in app form.
Or, again, phone based, both WAZE and Google Maps have inbuilt GPS speedometers, as well as informing you of the local speed limit.
If you’d like small, standalone, and the ability to use navigation, some members have used
Beeline Moto IIAnother option is
Road Angel, which is more of a speedometer with speed camera detector inbuilt.
Lastly, working on the same principle as the mobile phone apps, you can get digital gps speedometers, just look on eBay, Amazon, etc. They are similar and it will boil down to one you like the look of and size.