Dismantle the speedo assembly, lift the needle and move it 10. Carefully.
We put a secondhand speedo on one of our bikes and I rode it around doing 10 over for a year before I finally got a ticket. I argued with the cop too. After sticking the gps on it I found out he was right. Thats when I moved the needle.

 

A SpeedoHealer might work unless yours is a cable driven mechanical speedometer.

 

I have no idea what could be causing that because I have never worked on the speedo's on these bikes, but that pretty easy to live with! A lot better than calculating those percentages.

 

Thanks for the suggestion Michelle. I did this, it took a few goes but it is within a couple of kph
Cheers.

 

It could be you have a smaller than standard engine sprocket. ( the speedo is driven from the engine sprocket )
Some people fit them to give more better performance lower down the speed range.
I went the other way and fitted a larger one to give better miles per gallon at motorway speeds.
My speedo is now 5 mph slow. ( tested against a bike with a speedo driven off the front wheel ).
Enjoy the ride anyway !

 

Nope, speedo is driven from front wheel and I think some one may have played with it before. Works great now and very close at all speeds.
Cheers