  The Sydney University Giant Air-shower Recorder (SUGAR) measured the energy
spectrum of ultra-high-energy cosmic rays reconstructed from muon-detector
readings, while the Pierre Auger Observatory, looking at the same Southern sky,
uses the calorimetric fluorescence method for the same purpose. Comparison of
their two spectra allows us to reconstruct the empirical dependence of the
number of muons in the shower on the primary energy for energies between
$10^{17}$ and $10^{18.5}$ eV. We compare this dependence with the predictions
of hadronic interaction models \mbox{QGSJET-II-04} and \mbox{EPOS-LHC}. The
empirically determined number of muons with energies above 0.75 GeV exceeds the
simulated one by the factors $\sim$1.67 and $\sim$1.28 for $10^{17}$ eV proton
and iron primaries, respectively. The muon excess grows moderately with the
primary energy, increasing by an additional factor of $\sim 1.2$ for
$10^{18.5}$ eV primaries.
