Recently I got my android mobile, motorola fire xt. Such an amazingly bagged with all smart features. It has gps facility and by using custom software's we can use our phone as speedometer.
go to andriod market and search for speedview. Install it and open the software. activate gps and change settings according to ur wish from options.
You also have option to connect with maps from speedview settings.
This is a quick mobile post. Ill try posting detailed in future. Thanks