  We show that measurements of the forward-backward charge asymmetry
($A_{FB}(M,y)$) of Drell-Yan dilepton events produced at hadron colliders
provide a new powerful tool to constrain Parton Distribution Functions (PDFs).
PDF uncertainties are the dominant source of systematic error in precision
measurements at hadron colliders (e.g. $\chi^2$ values of fits to extract
$\sin^2\theta_{eff}^{lept}(M_Z)$ from $A_{FB}(M,y)$ with different PDF replicas
can be used to place additional constraints on PDFs.In turn, using these
constrained PDFs significantly reduces the PDF errors in precision measurements
of electroweak parameters. The measurement of the on-shell
$\sin^2\theta_{W}=1-M_W^2/M_Z^2$ is equivalent to an indirect measurement of
the W mass. The errors in this indirect measurement of the W mass are
competitive with direct measurements. For example, with 200 fb$^{-1}$ at 13
TeV, the expected error in the indirect measurement of the W mass is $\pm$9
MeV.
