Transmission ratios. How to determine gearbox model?

(Szerző: gm (Melegh Gábor), utoljára módosítva: 2007.01.12.)

Can the gearbox type/model be recognized by its casing?
Can I measure the ratio somehow on a gearbox not removed from the car?

Unfortunately the gearboxes are not labelled, all the covering parts of it have the same part number. The speedometer cable pinion is the easiest to remove, but forget that - its teeth number reflects to the differentaial ratio, not to the gearbox ratio(s). On a used Lada there can be any differentaial (1:3.9, 1:4.1, 1:4.3, 1:4.44), you cannot determine the gearbox of it.

The most informative method is to put the gearbox into 2nd gear, and turn the crank carefully, about 2 turns. (720 degrees). Before doing that, mark the driveshaft with a chalk. Now, you have about 2.00 turns on the crankshaft (gearbox input). Examine the driveshaft and slightly adjust the crankshaft while the driveshaft (gearbox output) shows exactly one turn. To do that, you have to creap under the car and back out a few times. 

1 turn on the driveshaft needs almost 2 turns on the crankshaft, if it was a 2106 gearbox.
2.1 means 2105,
2.3 means 2101.

I dont recommend removing the bottom cover of the gearbox. Theoretically you can count the teeth of the input drivewheel (28 teeth or 29 teeth). You can also examine the 1st gear gearwheel: 14 or 15 teeth. Practically this won't work since you cannot mark the starting tooth of your count. The greasy-oily-dirty mixture within the garbox and covering the cogwheels will clear any marks you make.

Can I replace my gearbox/diff to another model?

All RWD ("classic") differentials are intechangeable, including NIVA models.
All RWD ("classic") gearboxes are interchangeable (including NIVA) while keeping number of gears. (4 or 5). You can also replace a 4-speed box to an 5-speed one or vica versa, while replacing its rear crossmember and rubber mount too.

Classic Lada gearbox ratios:

2106 2107
 1  1:3.75  1:3.42  1:3.67  1:3.67
 2  1:2.30  1:1.989  1:2.10  1:2.10
 3  1:1.49  1:2.89  1:1.36  1:1.36
 4  1:1.00  1:1.00  1:1.00  1:1.00
 5  -  -  -  1:0.82
 R  1:3.87  1:3.340  1:3.53  1:3.53 

Classic Lada differential ratios:

2101, 21011: 1:4,3
2102: 1:4,47
2103: 1:4,1
2105/07: 1:4,1 or 1:3,9
2106: 1:3,9

Lada Samara gearbox ratios: (21081,21083, 2109, 2110)

I. 3.636
II. 1.95
III. 1.357
IV. 0.941
V. 0.784
R: 3.53