Could it be the speed sensor? I know my buddy ripped the speed sensor out of the tranny on his chevy gasser, and it was doing the same thing. I guess if its showing mph that wouldnt be it. Worth a check though, might be a bad connection. Be nice if it was something simple. good luck
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2005 White 2500 QC SB AT; 2" leveling kit, 35x12.5r17 BFG MT KM2s, no kitty, Magnaflow muffler, silencing ring MIA, K&N Intake w/ Amsoil filter, Front mount rec. hitch. Pricol boost, EGT, & trans temp.