At a stretch, you could use a bicycle speedo (Digital, ask for one that goes up to 200 km/hr or more and has miles as well as km's)
I did have one of them on my racebike for a while... records average speed, maximum speed etc as well...
When well caliberated will be more accurate than the stock one. Also good for bragging points (Mine can do 208k/hr. See?)
Or caliberate it off, and really brag to your mates (Mine does 280 in fourth. See!
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The Chopster