29% of the drive
The distance that you get reception is the length of the chord created by the intersection of the circle defining the edge of transmission and the line defining the car trip.
x2 + y2 = 532 this is the circle
y = -70x/73 + 70 this is the line
1. solve by substitution
2. determine the distance between two solutions (the intersections)
3. this is the distance during which the radio is heard.