In one trip of $m$ miles, the car travels $5,280m$ feet (since $1$ mile $= 5,280$ feet).

Each revolution of the tire covers a distance equal to the tire's circumference:

$C = \pi d$

The number of revolutions per trip is:

$\text{Revolutions} = \frac{\text{Total distance}}{\text{Circumference}} = \frac{5,280m}{\pi d}$