GPS speedometers are used typically for fun, or as backup when their regular speedometers are not working. A GPS is capable of reading the speed of your vehicle by recording how far the receiver has moved since the last measurement was taken. The speedometer accuracy is thus based upon a moving average, subject to the quality of the current satellite signal on the road. For this reason, GPS speedometers are helpful, but not as accurate as car speedometers.
Purchase a GPS system with a speedometer built in. TomTom and Garmin are two brands that include speedometers in their basic GPS devices.
Plug the GPS device into your car's power source and wait for it to acquire a satellite signal. This can happen instantly or take up to several minutes, depending upon your current location.
Find your GPS' speedometer, which is located in different places on different systems. On a Garmin, go into Settings, and then Speed; the speedometer will display on its own screen. On a TomTom, the speedometer is always displayed in the corner of whatever screen you have open.
Refer to your user's manual to set special features on your GPS unit's speedometer. TomTom units, for example, can be set to beep when you go more than 5 miles over the speed limit. You can also download extras such as TripMaster, which displays a large speedometer and provides other information, such as fuel consumption and cost per mile.
Save your settings, and start driving. Allow your GPS a few minutes to begin recording your speed.