Your scanmaster reads what the computer sends it through the ALDL. The vehicle speed sensor sends the speed info to a little yellow box under the dash.. that little box then sends the info to your ECM and your speedo(which is electric).
More than likely your speedo calibration is off. Maybe components inside it have changed value. The speedo is not TTA specific, you can get a speedo from a Firebird and swap it out see if it fixes the problem.
HTH
