I have bought a 91 Rally Sport with 305TBI and since I purchased it the speedometer has not worked. I replaced the entire gauge cluster with a new cluster that I knew worked. Where should I start to find the problem with my speedometer?

Well if the gauge fuse is good, I would start looking at the Vehicle Speed Sensor (VSS).

I've developed problems with my speedo also. Changed the gauge cluster, helped some, but the needle still bounces, especially under acceleration. Where is the VSS and what can a guy do to check it?
Thanks.

If your car has it, it will be mounted on the transmission and the buffer will be mounted under the dash on the passenger side near the ECM. Are you getting a code 24 (VSS circuit)

Sorry Trickster, I'm way too new at this to know anything about codes. I don't have a clue how to check for one. All I know is the speedo worked fine when I bought the car last year, then one day it just started jumping around after the first few minutes of driving. The new cluster almost fixed it, seems fine when I'm decelerating, but still jumps around a bit especially under heavy acceleration.