It's VERY common that stock gauges aren't accurate. That's part of the reason people go with aftermarket gauges. Tachs obviously being the most popular since those are the time critical elements. Since speedometers aren't usually important to racers, they're rarely replaced.
The signal they get from the computer is usually right, but the mechanical electronics and/or microprocessor in the factory gauges suck. Most GM tachs are as much as 500RPM off at any one time. LOL
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2002 Corvette Z06 - Totaled
2003 Corvette Z06 50th Anniversary
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