Muncie Four-Speed Specifications
Type	RPO	Model Year	Gear Ratios				Input Splines		Output	Counter	Main	Synch.	Synch.	Cluster	Helix
			1st	2nd	3rd	4th	Teeth	Grooves	Spline Teeth	shaft pindiameter	DriveTeeth	Hub Width	RingShldr.	Gear Teeth	GearAngle
Wide ratio	M20	1963-65	2.56	1.91	1.48	1.00	10	None	27	7/8	24	Wide	No	29	39
		1966-70	2.52	1.88	1.46	1.00	10	2	27	1	21	Narrow	Yes	25	39
		1971-74	2.52	1.88	1.46	1.00	26	2	32	1	21	Narrow	Yes	25	39
Close ratio	M20	1963-65	2.20	1.64	1.28	1.00	10	1	27	7/8	26	Wide	No	27	39
	M21	1966-70	2.20	1.64	1.28	1.00	10	1	27	1	26	Narrow	Yes	27	39
		1971-74	2.20	1.64	1.28	1.00	26	1	32	1	26	Narrow	Yes	27	39
	M22	1965-70*	2.20	1.64	1.28	1.00	10	None	27	1	26	Narrow	Yes	27	21
		1970**	2.20	1.64	1.28	1.00	26	None	32	1	26	Narrow	Yes	27	21
		1971-72***	2.20	1.64	1.28	1.00	26	None	32	1	26	Narrow	Yes	27	21
*All M22 applications except 454 Chevelle. **454 Chevelle applications. ***M22 transmissions were produced in 1973 and 1974 for other GM divisions and Chevrolet service applications only. The RPO column in the Muncie four-speed specifications chart designates how the transmission option was ordered. All Muncie wide ratio transmissions are considered M20s. All close ratio Muncies are considered either M21 or M22, depending on the gearset installed. Note that in 1963-65, the close ratio M21 had to be ordered under the RPO M20, with the factory deciding whether you received and M20 or M21 gearbox on the rear axle ratio specified. In 1966, the customer could designate transmission choice.