Scooters have terribly inaccurate speedometers so I at times use a Garmin Forerunner 201. Made for runners but works just fine for bikes. Displays and saves current speed, top speed, distance, average speed? and/or altitude depending on your settings. Plus you can download your data and overlay it onto Google Earth to see where you went...unnecessary but interesting.
be careful of a speedo that saves top .average speed etc,cause i am sure that in the event of an accident the police may look at it ,and if you have been speeding it would take some explaning
For example: http://scoot.net/gallery/bbs/route2.jpg