Consider a continuous culture provided with a sterile feed containing $10$ mM glucose. The steady state cell density and substrate concentration at three different dilution rates are given in the table below.

$$\begin{array}{|c|c|c|} \hline \text{Dilution rate} (h^{-1}) & \text{Cell density} (gL^{-1}) & \text{Substrate cconcentration} (mM) \\ \hline 0.05 & 0.248 & 0.067 \\ \hline 0.5 & 0.208 & 1.667 \\ \hline 5 & 0 & 10 \\ \hline \end{array}$$

The maximum specific growth rate $\mu _m$ (in $h^{-1}$), will be _________.