how to calibrate rb25 speedo

[rwp096]

i recently installed an rb25 transmission into my s14 with an rb20, the tranny didnt come with a speed sensor so i used one from a na 300zx and now my speedo reads roughly 40mph too fast. how do i go about calibrating the speedometer to read correctly?

[Zerolift Autolab]

Look up Dakota Digital - they make a Speedometer Interface box that wires into the car fairly easily.