I have a setup that I use to track my pace and distance during my trail runs that may actually be of some help to you - I'm sorry that it didn't come to mind earlier!
The system is the
Timex Ironman Triathlon Speed + Distance System. It consists of a GPS armband, and a wrist watch to which the GPS info is sent. Speed is among the many things that it monitors, but I think it only monitors in mph (although they may have a different setup for those in kph countries). Also, you'd want to make sure that it was set to actual speed, and not your speed average, or you might get into some trouble
The accuracy is excellent on mine. I've tried it on foot, bike, and auto, and as long as you're in satellite range the relay response time is outstanding, nearly immediate.
It's a little pricey at around 200.00, but I did find mine on sale for 69.00 at a sporting goods store, so you may be able to find it for less than retail. It is quite versatile, however, and if you hike, run, bike, swim, or sail, you'd be able to put it to good use elsewhere.
Hope that might be of help to you.