901 Basic Tech Data
Years Used: 1964-76
Models: 911, 912, 914
How to Interpret Gear Ratios:
Gear sets are made of at least 2 gears (1st gear in a five speed has 3 in the set). One gear is attached to its shaft and always spins with the shaft, and the other gear is able to spin independent of the shaft until selected.
Lets use AA gear set as an example; "12:34" means that the gear on the input shaft (drive gear) has 12 teeth, and the one on the pinion shaft (driven gear) has 34. The means that the input shaft has to spin much faster to get the out put shaft to rotate, which is what we expect in a first gear. Another way to annotate the gear ration is by dividing the driven gear/drive gear. In AA that gives a ratio of 2.833. The higher the number the lower the gear. As you look at the trend on the gears listed you will see that the further down the alphabet the lower the calculated ratio. The drive gear gains teeth and the driven gear loses them. The highest gear that can be made with stock sets is a flipped H gear moved from the 2nd gear spot in a 4 speed (3rd in a 5 speed) and run in reverse in the 5th gear position (32:19, 0.594). The reason you cannot go any higher than this is that the base circle of the driven gear is as small as possible. It cannot be made any smaller. To make an 18 toothed or smaller driven gear would take a smaller base circle than what the diameter of the pinion shaft could allow. The gears listed below do have some with 18 teeth or less, but those are in special spots like on a 904 shaft which has a smaller diameter where the special gears are mounted, or on gear that are machined directly onto the shaft as an integral part, like F and GA.
Gear Ratios:
1st gear: 11:34, 3.091 (A)
1st gear: 12:34, 2.833 (AA)
1st gear: 14:37, 2.643 (B)
1st gear: 12:29, 2.417 (C-)
1st gear: 15:36. 2.400 (C)
1st gear: 13:29, 2.231 (CA)
1st gear: 16:35. 2.188 (D)
2nd gear: 17:34, 2.000 (E) - *904 shaft only
2nd gear: 18:34, 1.889 (F) - Part of main shaft
2nd gear: 18:33, 1.833 (G)
2nd gear: 18:32, 1.778 (GA) - Part of main shaft
2nd gear: 17:30, 1.765 (GB)
2,3 gear: 19:32, 1.684 (H) - 2nd in 4 speed
2,3 gear: 19:31, 1.632 (HB) - 2nd in 4 speed
2nd gear: 20:32, 1.600 (HA) - *904 shaft only
2,3 gear: 20:31, 1.550 (I)
3rd gear: 21:32, 1.476 (J)
3rd gear: 21:30, 1.429 (K)
3rd gear: 22:31, 1.409 (KA) - Flipped ZD used backwards in 3rd
3rd gear: 22:30, 1.364 (L)
3,4,5 gear: 22:29, 1.318 (M)
3,4,5 gear: 23:29, 1.261 (N)
3,4,5 gear: 23:28, 1.217 (O)
3,4,5 gear: 23:27, 1.174 (P)
3,4,5 gear: 24:27, 1.125 (Q)
4,5 gear: 25:27, 1.080 (R)
4,5 gear: 25:26, 1.040 (S)
4,5 gear: 26:26, 1.000 (T)
4,5 gear: 26:25, 0.962 (U)
4,5 gear: 27:25, 0.926 (V)
4,5 gear: 28:24, 0.857 (X)
4,5 gear: 28:23, 0.821 (Y)
4,5 gear: 29:23, 0.793 (Z)
4,5 gear: 29:22, 0.759 (ZA)
4,5 gear: 31:22, 0.710 (ZD)
5th gear: 32:19, 0.594 (flipped H)
Ring and Pinion Options:
6:29 (4.83)
6:32 (5.33)
7:31 (4.43)
7:32 (3.86)
Factory LSD Info:
40-80% ZF friction disk "clutch type" differential was available and able to be fit into any 901 variant.
Standard Oil Info:
~2.5L of Hypoid SAE 90 was original recommendation. Non-synthetic GL5 80w90 is the appropriate modern substitution.
Interpretation of 901 series transmission serial numbers:
For 901, 902, 905, 911
Standard nomenclature
AAA/BB CC DEF GGGG
A = Type; 901, 902, 905, 911
B = Sub Type, Ex; 01, 80, 82, etc.
C = Differential type. 12 = ZF LSD, if blank then no LSD
D = Engine; 5 = 4cyl, 7 = 6 cyl
E = Unknown
F = Unknown
G = Serial number in sequence; 0001, 0002, and so on.
Using the above, interpretation of the following sample random serial number would be:
911/01 12 7xx 0911
A 911/01 transmission with an LSD differential, for a 6cyl engine, that was the 911th of that type stamped.
For 914/01 and 914/05 types used exclusively in 914-6:
Standard nomenclature
914/AA BB CDEFFFF
A – Type: 01 = Manual, 05 = Sportomatic
B – Differential: 10 = 80% LSD, blank = standard, no LSD
C – Application: 7 = 6-cyl engine
D – Type: 5 = 5 speed, 6 = 4 speed Sportomatic
E – Model Year: 0 = 1970, 1 = 1971, 2 = 1972
N – Production number
Using the above, interpretation of the following sample random serial number would be:
914/01 10 750 0914
A 914-6, with a manual transmission, an LSD, a 6 cyl engine (they all had that), 5 speed, model year 1970, 914th production stamping.
Early 914-4 transmissions; 914/11, some 914/12 from 1970 through early 1973:
Standard nomenclature
914/AA B CDEEEEEEE
A – Type: 01 = standard, 05 = Sportomatic
B – Differential: 10 = 80% LSD, Blank = standard, no LSD
C – Type: H = Manual
D – Gear/Differential: A = 5 speed, B = 5 speed + LSD
N – Production number
Using the above, interpretation of the following sample random serial number would be:
914/11 10 HB0009144
An early type 914/11, with LSD, manual transmission, again confirm LSD, 9144th production stamping.
Late 914-4 transmissions, 1973-1976:
Standard nomenclature
ABCCDDE
Note: there is not “Type” indication on these, they are all 914/12
A – Type: H = Manual
B – Gear/Differential: A = 5 speed, B = 5 speed + LSD
C – Day of manufacture
D – Month of manufacture
E – Year of manufacture
Using the above, interpretation of the following sample random serial number would be:
HB09115
A 914/12 transmission, manual, with LSD, manufactured 09 Sept 1975.