  The QCD coupling, $\alpha_s$, is not a physical observable since it depends
on conventions related to the renormalization procedure. Here we discuss a
redefinition of the coupling where changes of scheme are parametrised by a
single parameter $C$. The new coupling is denoted $\hat \alpha_s$ and its
running is scheme independent. Moreover, scheme variations become completely
analogous to renormalization scale variations. We discuss how the coupling
$\hat \alpha_s$ can be used in order to optimize predictions for the inclusive
hadronic decays of the tau lepton. Preliminary investigations of the $C$-scheme
in the presence of higher-order terms of the perturbative series are discussed
here for the first time.
