The rb20 cable is fine
take out the cable from the sender on the GB, with the car on ramps, and stands. Then start the car, put in gear and let out clutch. Take a look to see if inside the end of the sender spins (where cable goes into), when it in gear. If it does, then you may have a dodgy cable.
If it dont spin when the car is in gear with the clutch out,...if like me:D, you have not secured the r33 speedo sender properly (red cog on the end of the gtr cable drive unit), then it has slipped off, and is not sitting where it supposed to...( not reading speed from gb)
see my thread http://www.skylinesaustralia.com/forums/sh...ead.php?t=49866