Résumé
A new version of the optimized perturbation (OPT), implementing renormalization group properties consistently, allows to calculate the nonperturbative ratio of the pion decay constant and the basic QCD scale in the MSbar scheme. From the experimental Fπ input value it gives a new determination of $\Lambda_{MSbar}$ for nf=2 and nf=3, and thus of the QCD coupling constant $\alpha_S$ at various scales once combined with a standard perturbative evolution. The empirical convergence properties of the RGOPT modified series is exhibited up to the third order. We examine the various sources of theoretical uncertainties and obtain $\alpha_S(mZ)= 0.1174 ^{+.001}_{-.0005} \pm .001\pm .0005_{evol}$, where the first errors are estimates of the intrinsic theoretical uncertainties of our method, and the second errors come from present uncertainties in Fπ/F0, where F0 is Fπ in the exact chiral SU(3) limit.