You dont have to do anything to the transmission. The electronic speedometer just needs a signal. Once you wire it to a signal from a frequency source, you can program the speedo to read various speeds for any given frequency. So just wire the new speedo up to a wheel sensor or similar eg driveshaft sensor. Then calibrate the speedo for that signal. I did it on my 1994 e420 because I swapped the 2.24 for the 2.82. Obviously the speedo was going to be way off. There may be limits to the range of input frequencies it will accept; if you use something that spins a lot slower or more quickly than a driveshaft, then the speedo might not show anything, in which case a converter box would probably be required.