Changing the speedo is pretty straight forward, so I suggest that you do a simple test, switch the speedos, and do another test.
By test you need to do a control environment test.
Keeping your old speedo on the car, find an empty street and a strip of highway and a friend with his car/bike.
Do 3 tests each at a different speed. E.g. you do 40 while your friends vehicle is driving right next to you at the same speed and he records his speed. Repeat this for a few higher driving speeds.
Then change the speedo. You need to start with an allen key and the bolts along the sealing of the speedo pocket. Repeat the test and observe if the speeds indicated on your car are the same for your old and new speedos.