To find John's average speed in miles per hour (mph), you can use the formula:
\[ \text{Average speed} = \frac{\text{Total distance}}{\text{Total time}} \]
John runs a total distance of 6.0 miles in a total time of 2 hours. Plugging in these values:
\[ \text{Average speed} = \frac{6.0 \text{ miles}}{2 \text{ hours}} = 3.0 \text{ mph} \]
Therefore, John's average speed is 3.0 mph when rounded to the nearest tenth.