For an emissions test in a lab, stoichiometric ratio is useful. It means that the mixture contains exactly as much oxygen as needed to completely burn every molecule of fuel: no more, no less. The result is water and carbon dioxide only.
This is not what a car engine does (completely burn fuel, supra), and that number is not optimum for any particular RPM, vacuum, throttle position, spark position, or power level. Best WOT is always richer than stoich, cruising is leaner, it should vary throughout the range depending on conditions.