It's well known that the weakest link in the SAAB 900 is the Gearbox. The gearbox can not handle high torque.
But knowing this and driving with inside of the head, a stock gearbox could last until the car turns to scrap.

TOC
1. Specs
2. Primary gearing
3. Total gearing
4. Strengthening
5. Gearbox Oil

Specs
Number 1984-
example: GM 3 5 7 01

Pos 1:
Type
    GM = Manual
    GA = Automatic

Pos 2:
Gearbox fitting

Pos 3:
Gears
    3 = 3
    4 = 4
    5 = 5

Pos 4:
Gearing primary

Primary Gearing

 

Manual

Automatic
Primary 4 5 6 7 8 3 4
Tooth
in/out		 31/30 30/27 31/26 32/25 33/24 40/37 41/36
Gearing 0,97 0,90 0,84 0,78 0,72 0,93 0,88

Pos 5:
R&D version

Total Gearing
Below is a table that shows it on different gearboxes

Year Model Gearbox
number		 Final
gear		 Total gearing
1 2 3 4 5 reverse
1981 900 G34504 9:35
3.89		 13.59 7.68 5.15 3.50   14.95
1981 900 G45704 9:35
3.89		 13.77 7.78 5.22 3.76 3.04 15.15
1981 900 Turbo G44604 9:35
3.89		 12.66 7.16 4.80 3.26   13.90
1981 900 Turbo G45704 9:35
3.89		 13.77 7.78 5.22 3.76 3.04 15.15
1982 900 G34406 9:33
3.67		 13.78 7.79 5.23 3.55   15.15
1982 900 Turbo G45606 9:33
3.67		 13.94 7.88 5.29 3.80 3.08 15.34
1983 900GL G45704 9:33
3.67		 13.78 7.79 5.23 3.55   15.15
1983 900GL, GLs, Turbo G45606 9:33
3.67		 13.94 7.88 5.29 3.80 3.08 15.34
1983 900GLi, EMS, GLE G45706 9:33
3.67		 12.99 7.34 4.93 3.54 2.86 14.28
1984 900 GL G34506 9:33
3.67		 12.81 7.24 4.86 3.30   14.09
1984 900GL, GLi, GLs, GLE G45706 9:33
3.67		 12.99 7.34 4.93 3.54 2.86 14.28
1984 900 Turbo G45606 9:33
3.67		 13.94 7.88 5.29 3.80 3.08 15.34
1985 900GL GM34506 9:33
3.67		 12.81 7.24 4.86 3.30   14.09
1985 900, GL, Turbo GM45606 9:33
3.67		 13.94 7.88 5.29 3.80 3.08 15.34
1985 900 Turbo 16 GM45706 9:33
3.67		 12.99 7.34 4.93 3.54 2.86 14.28
1986 900GL GM34506 9:33
3.67		 12.81 7.24 4.86 3.30   14.09
1986 900GL, i, Turbo GM45606 9:33
3.67		 13.94 7.88 5.29 3.80 3.08 15.34
1986 900 Turbo 16 GM55712 9:33
3.67		 12.99 7.34 4.93 3.54 2.86 14.28
1987 900i, Fi GM34408 9:33
3.67		 13.78 7.79 5.23 3.55   15.15
1987 900c GM34508 9:33
3.67		 12.81 7.24 4.86 3.30   14.09
1987 900c, i, i16, S16, T8,
i cat		 GM45610 9:33
3.67		 13.94 7.88 5.28 3.80 3.07 15.33
1987 900 Turbo 16 cat GM55602 9:33
3.67		 13.94 7.88 5.28 3.80 3.07 15.33
1987 900i cat GM45710 9:33
3.67		 12.99 7.34 4.93 3.54 2.86 14.28
1987 900 Turbo 16 GM55710 9:33
3.67		 12.99 7.34 4.93 3.54 2.86 14.28
1988 900i, Fi GM34408 9:33
3.67		 13.78 7.79 5.23 3.55   15.15
1988 900c GM34508 9:33
3.67		 12.81 7.24 4.86 3.30   14.09
1988 900c, i, i16, S16, T8,
i cat		 GM45610 9:33
3.67		 13.94 7.88 5.28 3.80 3.07 15.33
1988 900 Turbo 16 cat GM55602 9:33
3.67		 13.94 7.88 5.28 3.80 3.07 15.33
1988 900i cat GM45710 9:33
3.67		 12.99 7.34 4.93 3.54 2.86 14.28
1988 900 Turbo 16 GM55710 9:33
3.67		 12.99 7.34 4.93 3.54 2.86 14.28
1988 900i cat GM45510 9:33
3.67		 14.96 8.46 5.67 4.08 3.30 16.46
1989 900i cat, i16 cat GM45612 9:35
3.89		 14.85 8.64 5.66 4.09 3.26 16.34
1989 900c, i, i16, S16, T8,
T16 cat		 GM45712 9:35
3.89		 13.84 8.05 5.27 3.81 3.04 15.22
1989 900 T8, T16,
T16 cat, T16S		 GM55704 9:35
3.89		 13.84 8.05 5.27 3.81 3.04 15.22
1989 900 T8, T16,
T16 cat, T16S		 GM55804 9:35
3.89		 12.88 7.49 4.91 3.55 2.83 14.17
1990 900i cat, i16, i16 cat GM45612 9:35
3.89		 14.85 8.64 5.66 4.09 3.26 16.34
1990 900c, i, i16, S16 GM45712 9:35
3.89		 13.84 8.05 5.27 3.81 3.04 15.22
1990 900 T16 cat GM55704 9:35
3.89		 13.84 8.05 5.27 3.81 3.04 15.22
1990 900 T16 GM55804 9:35
3.89		 12.88 7.49 4.91 3.55 2.83 14.17
1991 900i16, i16(2.1), S16 GM45614 9:35
3.89		 14.91 8.56 5.68 4.10 3.36 16.40
1991 900, i16(2.1), T16 cat GM45714
GM55706		 9:35
3.89		 13.89 7.98 5.29 3.82 3.04 15.27
1991 900 T16 GM55806 9:35
3.89		 12.92 7.42 4.92 3.55 2.83 14.21
1992 900i16, i16(2.1), S GM45614 9:35
3.89		 14.91 8.56 5.68 4.10 3.36 16.40
1992 900, i16(2.1), T16 cat GM54714
GM55706		 9:35
3.89		 13.89 7.98 5.29 3.82 3.04 15.27
1992 900 T16 GM55806 9:35
3.89		 12.92 7.42 4.92 3.55 2.83 14.22
1993 900i16, 900i i16(2.1), S GM45614 9:35
3.89		 14.91 8.56 5.68 4.10 3.36 16.40
1993 900, i16(2.1), T16 cat GM54714
GM55706		 9:35
3.89		 13.89 7.98 5.29 3.82 3.04 15.27
1993 900 T16 GM55806 9:35
3.89		 12.92 7.42 4.92 3.55 2.83 14.22

The following info is supplied by Dennis, Who works at EBB. EBB are SAAB specialists.
Please visit them.

HTTP://www.ebb.se

Strengthening
The most durable alternative is a chill cast (kokill) 4-speed gearbox. With a change of Pinion bearing and making of a steel cylinder that replaces the original distance ring in between the bearings. Due to this mod the pre-torque of the bearings should not change.
Then use a 32/25 primary gear. This will result in gear box that should manage up to aprox 230 BHP.
A 5 speed gearbox is possible to mod in the same way. But due to the different model the gearbox is more flexible in the casing. When flexing the 3:rd gear will get broken! The tooth of the gear will just break.

Gearbox Oil
SAAB Has for years recommended motor oil for the gearboxes. This is just madness. (would you use gearbox oil instead of motor oil? Shouldn't think so!) The bearings and the end gear in the SAAB gearbox needs a real gearbox oil.
SAAB recommended the motor oil due to the gearbox would be easier to shift gears in. Then they forgot that it takes some special lubrication for the bearings and such, to keep them from break before theirs normal wear. Hence all  900/9000/900NG with broken pinion bearings.
Then changing the oil is important. But everyone has not understand this yet, not even some of the larger SAAB workshops.
There will always be some particles from the gears, Sync-rings, etc. So, the oil should be changed at least each 50000-60000 km.
If this is not done, the particles will stick to the bearing surfaces and "presto" the bearings will be worn out.

Dennis recommends two different types of Quaker State gearbox oils. He has great experience with these and tests has proven that they are great.

- For daily use (stock, no racing) use Qudralube 75W

- For hard tuned cars and racing use QS2105

The gearboxes will be very easy-shifted and they will last longer.