GM has finally corrected the bug that existed since 1980s as to maximum MPH of 158 for OBD-II for those with C6s
as seen below for the PCM there is a update to PCM software to finally report above that speed without tripping a DTC. Your PCM would require a GM update if it does not have this fix.

Part Number CVN Description
12614682 0000DB13 New software contains diagnostics enhancement for DTC P0128 and DTC P0507. Also addresses battery drain.

12605732 0000793A New software to ensure proper catalyst monitoring.

12605196 N/A New software to eliminate erroneous "Max speed 158 mph" DIC message.

12602922 000016FB New software to correct problem occuring at the assembly plant. Does not address any service issues.

12601696 00004C95 New software for start of production

12600132 000079AE Main Operating System