Calibrating speedos is tricky work. Getting it apart and back together is half the battle. The little Huret speedo on the motobecane has a chrome bezel, with little bent tabs that hold it in place. Getting these tabs skillfully unbent and rebent requires a careful touch..
I've never looked inside a Huret, but most simple speedos have a hairspring that is attached to the needle shaft on one end, and a moveable plate on the other. You rotate the moveable plate to increase or decrease the tension on the spring. Normally, more tension = lower reading.
Since you probably don't have a dyno to use for calibration, about all you can do is make a small change, and check the results. You might have to make several adjustments before you get it correct.
If you have any of those "You are driving this fast" radar display signs in your area, that might be a good way to check it.
Hope this helps.