The format for the 7 inch disk to be played in the car was 16 rpm (16 2/3 rpm)
THE LONG GONE, BUT OFTEN WONDERED ABOUT 16rpm SPEED
The information above comes from the excellent book by Jim Dawson & Steve Propes
Grab your own copy HERE
The history of the 45 from it's creation to it's demise.
Jumpin' George Oxford radio commercial from 1955; 45s or 78s?