Believe there is a firm called 'redial' out there. Have a search on the web...

Or try a Motrax mechanical converter - a little gearbox which slows the cable drive down so your speedo reads in MPH.
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball