```
acceleration = (final velocity - initial velocity) / time
```
In this case, the initial velocity is 0 miles per hour and the final velocity is 55 miles per hour. The time taken to reach the final velocity is 6 seconds.
Substituting these values into the formula, we get:
```
acceleration = (55 mph - 0 mph) / 6 s
acceleration = 55 mph / 6 s
acceleration = 9.17 mph/s
```
Therefore, the acceleration of the car is 9.17 miles per hour per second.