Speedo gearbox MK2/MK3

Having installed a MK3 disc brake wheel on my MK2 (with the MK3 speedo gearbox), I discovered that my speedo is now very very optimistic... is it possible to put back my MK2 speedo gearbox, upside down, on the left or is it possible to put a MK3 speedo in a MK2 cup? Or//
any idea welcome!
Phil

The speedometers are the same ratio. Only the gearbox is different. If you have the proper gearbox, the problem must be in the speedometer. The gearbox should be a 15/12 ratio.