There has been much written about this, but I think this chart prepared by Grease Monkey is the definative work on the stock Z8's mph/rpm ratios.
Below are his calculations
*Take the RPM shown on the tach, divide by the transmission gear ratio, then divide that answer by the differential ratio.
*Set that number aside.
*Now divide 63,360 (inches in a mile) by the circumference of the rear tire in inches and set that number aside. *Finally, divide the first number by the second number and multiply by 60.
You now have the speed you are traveling in MPH.