haven't used nokia since Android came out, but there must be an app store from where you download apps (free and paid both), try searching for "speedometer" named apps and pick up any free one which comes on top with good comments about it. then turn on your GPS, launch the app when you are on the move and it will start showing your speed, location, direction everything...
GPS shows real speed not less speed. It is actually the meters which show more than actual speed, and mostly our bikes meters tend to be like this. I have matched speedometer on my HTC with corolla 2001 and it was showing almost exactly same speed as car's meter (hardly 1~2Km/h difference)