On the OG9-3 (1998-2002+2003CV) and 9-5 (1998-2010) the Tech-2 can program the Main Instrument Unit (MIU) with various tire sizes, so the speedometer is pretty close to right.
Gather information of which tire size you are using.
MIU (Main Instrument Unit)
Manualto enter the dynamic tire circumference manually.
OKto program the new value.