To find the unit rate of miles per hour, you can divide the total miles driven by the total hours taken. Mr. Stone drove 320 miles in 5 hours, so the calculation is as follows:
\[ \text{Unit Rate} = \frac{320 \text{ miles}}{5 \text{ hours}} = 64 \text{ miles per hour} \]
Therefore, the unit rate is \( 64 \) miles per hour.