Xt 600 e speedo fault

[xtcommuter]

Hi all, my bike has developed a fault with the speedo, essentially although the needle moves, it bares no resemblance to the speed I am doing, at best it will not hit more than 40 mph. The only variant I can think of is that it was originally an imported bike from Europe and the previous owner has put a UK face on it. Mileage covered is km but has the advantage that I know what speed I am doing in UK without having to do the mental maths. Has worked ok for six months so don't think that is the issue. Any advice / suggestions appreciated.
Many thanks in advance
Ian