Couldnt speak for a sprinter one in particular, but all the speedos I have had apart the needle just pulls off. Just grip the needle firmly between thumb and index finger and pull off. Ensure that you pull back square or else you could bend the shaft the needle fits to. DO NOT USE A SCREWDRIVER UNDER ONE SIDE OF THE NEEDLE as this will bend the shaft for sure.
Just check that the numbers are in the same place around the gauge back panel, if they are then swap the entire speedo mechanism (i.e. just change the back panel over). It would be a lot easier than stuffing about with the gears.