Sounds like something may have been missed in the tune, a cell or table. If the CEL is flashing it's a missfire from something, has nothing to do with speedo.
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1999 Z28 Camaro|Cut up
2003 CC Duramax Dually|19.5 w/245/70/19.5|5in exhaust
2009 G8 GT 408hp/402tq sae|Rotofab CAI|LS9 Cam|Headers|GXP Mufflers|Custom System
2009 Tahoe stock
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