I have a 91 Z28, L98 car. The vehicle has a TH350 swapped in. It looks like the stock tailshaft from the 700R4 was reused on the new trans. I bought the vehicle from a friend who claims that the speedo worked when he had it running (3 years ago). Ever since I've had the car the speedo has read right around 1/2 what it should, the odometer functions accordingly. My gauge cluster has some intermittent bad ground issues I havent tracked down yet, mostly to do with backlighting ground and the oil pressure gauge going in and out, (the old "hit the dash" method usually corrects these) not sure if it matters or not.

Any ideas on my speedometer woes? Is the VSS a simple magnetic pickup on these cars? I have access to a oscilloscope, but am not sure what the proper signal would look like

 

The VSS signal actually goes to the ECM first in TPI cars. Then the ECM sends a signal to drive the speedometer.
I'd check to see what the ECM is reading for MPH using a scanner first. It is possible with burning your own PROMs to adjust and correct it.

 

Thanks very much for the reply- I forgot that the computer has to figure out when to lock the convertor. I'll plug in the scanner tomorrow and see whats going on.

 

could just be that the gauge panel needs to be tightened.. or the cluster harness connections need to be cleaned up a bit.