I have a GPS Speedo on the steering column. It agrees with roadside signs all the time and reads around the same as the speedo.
If you use self adhesive velcro on the steering column behind the dashboard and fabricate a bracket from an old tupperware box, you can make quite a neat unobtrusive fastening that is easy to remove when the time comes for the car to be sold. I power it from a USB socket fitted in the radio blanking plate.
