79 Z28 the speedometer shows more then 20percent less then correct speed. Changed the gear driven from red to brown, now it is faster, but still much to slow, minimum 10 km/h, the more the faster the car is going. It is all factory setup, turbine wheels in 15 with 225/70 tires on, no modification in trans or rear end. Couldn't imagine the cable is the cause, could there be any internal problem in the speedometer unit? No more idea on my side, thanks for any comments, Uwe