dedge

I had my Z28 on the dyno last week and they told me that my speed sensor was bad after the first pull by looking at a graph that I guess monitored the different sensors and displayed them all on one screen. Where is this sensor located? I have also been having problems at the drag strip with the car experincing problems banging the rev limiter (at times but not always) between 1st and 2nd gear. But when it came off the dyno last Friday I road tested the car and it ran fine. Anybody know anything about this? Could these problems be related?