Not sure which censor you swapped on the transmission but you might want to take a look at the VSS sensor on the top of the rear pumpkin. When they go, and they often go, they usually mess with the speedo. Just get down there with a wrench (10mm I believe) and pull the single bolt and check the sensor. Clean it off and put it back in then test to see if the speedo works. Or a replacement sensor and napa is around $25.
Could be a cut in the line between the sensor and the dash or even a dead gauge. Don't know too much how to test for this, hopefully someone else with the knowledge can help you out here; but the VSS is always a good place to start.
Any warning lights?