The circumference $C$ of a circle is related to its diameter $d$ by the formula:

$C = \pi d$

Given that the circumference $C$ is 12.56 miles, we can find the diameter by rearranging the formula:

$d = \frac{C}{\pi}$

Substituting the given value:

$d = \frac{12.56 \text{ miles}}{\pi} \approx \frac{12.56 \text{ miles}}{3.1416} \approx 4 \text{ miles}$

So, the diameter of the circle is approximately 4 miles.

Would you like more details or have any questions? Here are some related questions you might find interesting:

1. What is the radius of the circle with a circumference of 12.56 miles?
2. How do you calculate the area of a circle if you know the diameter?
3. If the circle's radius were doubled, what would be the new circumference?
4. How is the diameter of a circle related to its area?
5. What is the length of the arc subtended by a 90° angle in this circle?

Tip: Always remember, the diameter is twice the radius of a circle!