This is an estimate, not a time slip
The formulas are empirical shortcuts that relate horsepower to race weight. They can be useful for sanity checks and bench racing, but a real pass depends heavily on traction, launch, gearing, shift strategy, aerodynamics and weather.
Do not use this estimator to predict safe speeds or vehicle capability on public roads. Performance testing belongs in a controlled environment.
Crank horsepower versus wheel horsepower
For comparisons, use the same horsepower basis across vehicles. Mixing crank horsepower for one car and wheel horsepower for another makes the comparison misleading.