  We calculate the ratios $R_{\tau / P}\equiv \Gamma \left( \tau \to P
\nu_{\tau}[\gamma] \right) / \Gamma \left(P\to \mu \nu_{\mu}[\gamma]\right)$
($P=\pi,K$) at one loop following a large-$N_C$ expansion where Chiral
Perturbation Theory is enlarged by including the lightest resonances and
respecting the short-distance behavior dictated by QCD. We find $\delta
R_{\tau/\pi}=(0.18\pm 0.57 )\%$ and $\delta R_{\tau/K}=(0.97\pm 0.58 )\%$,
where the uncertainties are induced fundamentally by the counterterms. We test
the lepton universality, obtaining $\left|g_\tau/g_\mu\right|_\pi=0.9964\pm
0.0038$ and $\left|g_\tau/g_\mu\right|_K=0.9857\pm 0.0078$, and analyze the CKM
unitarity, getting results at $2.1\sigma$ and $1.5\sigma$ from unitarity via
$|V_{us}/V_{ud}|$ and $|V_{us}|$, respectively. We also update the search for
non-standard interactions in $\tau$ decays. As a by-product, we report the
theoretical radiative corrections to the $\tau \to P \nu_{\tau}[\gamma]$ decay
rates: $\delta_{\tau \pi} = -(0.24 \pm 0.56) \%$ and $\delta_{\tau K} = -(0.15
\pm 0.57) \%$.} \maketitle
