Just looked at the interior pictures of the Z06 on Chevys website and noticed something interesting about the speedometer.

From 0 to 100 MPH, the difference in readings for a given distance traveled by the speedometer needle is in 10 MPH increments. After 100 MPH, the speedometer needle is in 20 MPH increments for the same given distance between readings.

As you can imagine, accuracy would go right down the toilet unless the speedometer is recalibrated between the different incremental ranges by the computer.

Does anyone here know if this is the case?

It's an electronic gauge, basically using a stepper motor so they just re-program it above 100 to step only half as far per speed pulse. Potentially its dead accurate if they calibrate it well below 100.

The C5 guages are the same way (the increments change past a certain speed)