a bicycle trip computer
If reseting your speedo or tripmeter doesn't work, and you don't want a gps or navigator atached to your bike.
The cheapest and easiest way is finding a cheap bicycle trip computer. Most of them can display speeds above the 100km (or mph) and has several displaymodes (speed/tripcounter/daycounter/stopwatch etc).
You don't have to go for the more expensive ones (like topeak, trailtech or sigma) even the ones found at those wholesales (eurostores or dimestores) or other outlets can perfectly fit your needs for a few euro's/dollars/pounds.
Find a place to fit the magnet and sensor place the computer somewhere (on the dash?) calibrate and start touring.
If you google there is alot of info on the internet about using this kind of cycle-computer on other vehicles than bicylcles (they are even used as lowbudget triptometers on rallies).(for instance
after a 2 minutes search)
Only drawback can be no backlight in the dark, but I'm sure those excists also (some are even wireless)
bart & sophie