Hi all...
I've just found out, from The Saab Forum that this fault is most likely caused by the ABS sensor attached to the rear nearside (passenger side) wheel. There are sensors on all 4 wheels but this one controls the speedo.
Apparently it is integrated into the hub but you can try cleaning it and the toothed wheel, which might help.
Failing that, it's a new sensor
                
 Colin
                
 March 2010