To determine the fastest average speed, we calculate the speed for each interval by using the formula: speed = distance/time.
For the intervals:
- [0, 4]: \( \frac{248 - 0}{4 - 0} = 62 \) mph
- [4, 7]: \( \frac{449 - 248}{7 - 4} = 67 \) mph
- [7, 9]: \( \frac{565 - 449}{9 - 7} = 58 \) mph
- [9, 12]: \( \frac{757 - 565}{12 - 9} = 64 \) mph
The fastest average speed was during the interval [4, 7], with an average speed of 67 mph, as it has the highest calculated speed among all intervals.