To find the unit rate of miles per hour, you can divide the total miles driven by the total hours it took.
Mr. Stone drove 320 miles in 5 hours:
\[ \text{Unit Rate} = \frac{\text{Total Miles}}{\text{Total Hours}} = \frac{320 \text{ miles}}{5 \text{ hours}} = 64 \text{ miles per hour} \]
Therefore, the unit rate is 64 miles per hour.