Well, you guys may think this is a simple calculation, but it is not.
First off, everybody is ignoring the aspect of tire slippage vs. tire growth.
More importantly, however, are some additional factors commonly left out of
1. Accuracy of tachometer
2. Actual memory of the tachometer reading
3. Ability to read the tachometer, under hard braking, setting up for
corner #1 at Road America, realizing you're on the wrong line
4. Attempting to read the tachometer through trifocals while going over the
speed bumps (not a factor if you're under 50 yrs old)
5. Fleeting memory of the last incident you saw at this corner
6. Growing suspicion that the wet spot in the drivers suit is not perspiration
7. Proximity of members of opposite sex when telling others how fast you