Transmission Reduction Ratios

Columns are defined below:

Ratios

Transmission input shaft revolutions to 1 revolution of wheel.

Overall Min Reduction / Overall Max Reduction 

Crankshaft revolutions to 1 revolution of wheel.

Gear identification

Letters A, B, C, D in the diagram match table columns

Ratio Formula

Total Gear Reduction = (B/A) * (D/C)

Double Gear Reduction Models

Model	A	B	C	D	Ratio	Variator Ratio	Overall Min Reduction	Overall Max Reduction
1983/84 NB50 Aero	14	33	13	44	7.978	2.4 - 1.2:1	9.58	19.17
1985 NB50 Aero	12	34	12	47	11.097	2.4 - 0.8:1	8.88	26.63
1986-87 NB50 Aero	12	34	12	47	11.097	2.4 - 0.8:1	8.88	26.63
87 SE50 A	13	34	12	47	10.244	2.4 - 0.8:1	8.19	24.58
87 SE50P A (restricted)	12	34	12	47	11.097	2.4 - 1.1:1	12.21	26.63
87 SE50P AA (restricted)	12	34	12	47	11.097	2.4 - 1.1:1	12.21	26.63
88-93 SA50 Elite	12	34	12	47	11.097	2.4 - 0.8:1	8.88	26.63
94-up SA50 Elite	12	42	13	45	12.115	2.45 - 0.85:1	10.30	29.68
SA50P (restricted)	12	42	13	45	12.115	2.40-1.15:1	13.93	29.08

Aftermarket gears giving the ratios listed have been reported in the below combinations. Full Thread

Aftermarket Gears	A	B	C	D	Ratio	Variator Ratio	Overall Min Reduction	Overall Max Reduction	Notes
C & D gear swap (secondary)	12	42	15	42	9.80	2.45 - 0.85:1	8.33	24.01
	12	42	16	42	9.19	2.45 - 0.85:1	7.81	22.51
	12	42	16	41	8.97	2.45 - 0.85:1	7.62	21.97
	12	42	17	41	8.44	2.45 - 0.85:1	7.18	20.68
	12	42	18	41	7.97	2.45 - 0.85:1	6.78	19.53
	12	42	18	39	7.58	2.45 - 0.85:1	6.45	18.58
	12	42	19	37	6.82	2.45 - 0.85:1	5.79	16.70
	12	42	20	36	6.30	2.45 - 0.85:1	5.36	15.44
A & B gear swap (primary) AF16	13	35	13	45	9.320	2.45 - 0.85:1	7.92	22.83
	15	33	13	45	7.61	2.45 - 0.85:1	6.47	18.65
	12	24	13	45	6.92	2.45 - 0.85:1	5.88	16.96
	15	30	13	45	6.92	2.45 - 0.85:1	5.88	16.96
	16	32	13	45	6.92	2.45 - 0.85:1	5.88	16.96
	16	30	13	45	6.49	2.45 - 0.85:1	5.52	15.90
A & B gear swap (primary) AF05	16	32	12	47	7.83	2.40 - 0.80:1	6.27	18.80	Malossi 67-6458

Sym DD50

Aftermarket Gears	A	B	C	D	Ratio	Variator Ratio	Overall Min Reduction	Overall Max Reduction	Notes
Stock DD50 Primary/Secondary	12	34	12	47	11.1	2.45 - 0.85:1	9.43	27.19
Stock DD50 Pri/ Dio stock Sec	12	34	13	45	9.81	2.45 - 0.85:1	8.34	24.03
This row & below, Stock DD50 Primary/Dio aftermarket gears	12	34	15	42	7.93	2.45 - 0.85:1	6.74	19.44
	12	34	16	42	7.44	2.45 - 0.85:1	6.32	18.22
	12	34	17	41	6.83	2.45 - 0.85:1	5.81	16.74
	12	34	18	41	6.45	2.45 - 0.85:1	5.49	15.81
	12	34	18	39	6.14	2.45 - 0.85:1	5.22	15.04
	12	34	19	37	5.52	2.45 - 0.85:1	4.69	13.52
	12	34	20	36	5.10	2.45 - 0.85:1	4.34	12.50

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Ratio Formula

Reduction = (B/A)

Single Gear Reduction Models

Model	A	B	Ratio	Pulley Ratio	Overall Reduction
88 SB50, SB50P	13	83	6.385	1.8:1	11.49
1984-85 NQ50 A, AA Spree	12	83	6.917	1.8:1	12.45
1986-77 NQ50 A Spree	13	83	6.385	1.8:1	11.49
1986-87 NQ50 AA Spree	12	83	6.917	1.8:1	12.45
Aftermarket	15	81	5.40	1.8:1	9.72

-- Authors: Mousewheels, Wheelman, 808BMW