If you have a GPS enabled device and want an accurate speed meter, you need to try the free ans simple GPS Speedo application from XDA Developers. Click on to read more.

It will use GPS signal to display the current speed at which the device is currently moving. The application is at an initial version which currently features user configurable background colours, Day and Night colour schemes, Portrait and Landscape modes with gesture recognition to move between screens. It has been tested on HTC Touch HD and Diamond but according to the author whould work on any GPS enabled Windows Phone. You need to have .NET Compact Framework 3.5 installed in order to run this app.