  Measurements of the $R_{D^*}\equiv\mathrm{Br}(B\rightarrow \tau\nu
D^*)/\mathrm{Br}(B\rightarrow e\nu D^*)$ parameter remain in tension with the
standard model prediction, despite recent results helping to close the gap. The
standard model prediction it is compared with considers the $D^*$ as an
external particle, even though what is detected in experiments is a $D\pi$ pair
it decays into, from which it is reconstructed. We argue that the experimental
result must be compared with the theoretical prediction considering the full
4-body decay $(B\rightarrow l\nu D^* \to l\nu D\pi)$. We show that the
longitudinal degree of freedom of the off-shell $D^*$ helps to further close
the disagreement gap with experimental data. We find values for the ratio
$R_{D\pi}^l \equiv {\mathrm{Br}(B\rightarrow \tau\nu_\tau D
\pi)}/\mathrm{Br}(B\rightarrow l\nu_l D\pi)$ of $R_{D\pi}^e=0.274\pm0.003$ and
$R_{D\pi}^\mu=0.275\pm0.003$, where the uncertainty comes from the uncertainty
of the form factors parameters. Comparing against $R_{D\pi}$ reduces the gap
with the latest LHCb result from $1.1\sigma$ to $0.48\sigma$, while the gap
with the latest Belle result is reduced from $0.42\sigma$ to just $0.10\sigma$
and with the world average results from $3.7\sigma$ to $2.1\sigma$. Erratum
added at the end of the file.
