Green’s Dictionary of Slang

(US black) a car; thus fly crutch, an expensive, fashionable car; P-crutch, a police car.

1938		‘Sl. among Nebraska Negroes’ in AS XIII:4 Dec. 316/2: A crate or crutch is an automobile.
1960, 1975		Wentworth & Flexner DAS 133/1: crutch n. An automobile.
1994		C. Major Juba to Jive.