All '89-'94 M5s were
equipped with the Getrag 280 gearbox. This is a good,
robust gearbox that follows it's lineage back through the E28
M5's virtually identical 280, and the 265 and 260 used in the
535i, 533i, and 6-series variants. Despite a propensity
to get a bit "crunchy" in 2nd gear, it can take a
lot more power transmitted through it that BMW ever
supplied. The 280 in my 475hp E28 M5 Turbo felt as good
at 35K miles as any brand-new unit.
The differentials in M5s varied by market. '89-'94 cars
sold outside of the USA had the 3.91 limited slip
differential. This diff, coupled with the 5-speed's gear
ratios, provided a good spread in ratios and reasonable
highway cruising.
In the USA, only the '91 cars had the 3.91. Apparently
for fuel economy reasons, the 3.73 gearset was used in the
'92-'93 cars. Why "fuel economy" was a factor
in the country that has almost the lowest gas prices in the
world is a bit of a mystery.
In 1995, the last year of
E34 production, BMW fitted the Getrag 280/6 6-speed
transmission to the M5. The gear ratios are completely
different than the 5-speed 280, and the differential gearset
was changed to the 3.23 to take advantage of the change.
Instead of installing a close-ratio box, the net effect of the
6-speed/3.23 combo is simply an additional overdrive
gear. 1st-5th are basically the same as the 5-speed/3.91
combo, and the 6th allows low-rpm cruising. A
performance change? No. This was done strictly for
fuel economy as well.
Nevertheless, this does not preclude the possibility of
re-configuring a few parts to create the best of both
performance and economy worlds; enter the 6-speed and either 3.45 or 3.64
combination. These are not common gearsets, the 3.45 comes
from various older 7-series models and the 3.64 from the 8-series. Thes gearsets
are completely compatible with the M5's differential housing.
I originally used the 3.45, but eventually change to the 3.64. First, compare the numbers for the factory combinations and my
combinations:
5-speed
with 3.73 |
TRANS
& DIFF |
GEAR |
OVERALL
GEAR |
MPH @
3000 RPM |
MPH @
7250 RPM |
|
1st |
3.51 |
13.09 |
17 |
40 |
|
2nd |
2.08 |
7.76 |
28 |
68 |
|
3rd |
1.35 |
5.04 |
44 |
105 |
|
4th |
1 |
3.73 |
59 |
142 |
|
5th |
0.81 |
3.02 |
73 |
175 |
|
|
|
|
|
|
5-speed
with 3.91 |
TRANS
& DIFF |
GEAR |
OVERALL
GEAR |
MPH @
3000 RPM |
MPH @
7250 RPM |
|
1st |
3.51 |
13.72 |
16 |
39 |
|
2nd |
2.08 |
8.13 |
27 |
65 |
|
3rd |
1.35 |
5.28 |
42 |
100 |
|
4th |
1 |
3.91 |
56 |
135 |
|
5th |
0.81 |
3.17 |
69 |
167 |
|
|
|
|
|
|
6-speed
with 3.23 |
TRANS
& DIFF |
GEAR |
OVERALL
GEAR |
MPH @
3000 RPM |
MPH @
7250 RPM |
|
1st |
4.23 |
13.66 |
16 |
39 |
|
2nd |
2.53 |
8.17 |
27 |
65 |
|
3rd |
1.67 |
5.39 |
41 |
99 |
|
4th |
1.23 |
3.97 |
56 |
135 |
|
5th |
1 |
3.23 |
69 |
166 |
|
6th |
0.83 |
2.68 |
83 |
200 |
|
|
|
|
|
|
6-speed
with 3.45 |
TRANS
& DIFF |
GEAR |
OVERALL
GEAR |
MPH @
3000 RPM |
MPH @
7250 RPM |
|
1st |
4.23 |
14.59 |
15 |
37 |
|
2nd |
2.53 |
8.73 |
25 |
61 |
|
3rd |
1.67 |
5.76 |
38 |
93 |
|
4th |
1.23 |
4.24 |
52 |
126 |
|
5th |
1 |
3.45 |
64 |
155 |
|
6th |
0.83 |
2.86 |
77 |
187 |
As shown, the original equipment setup for
the 6-speed/3.23 is effectively the same as my car's original
5-speed/3.91 combination. By building a diff with the
3.45 gearset, the effective result is a 7% lower 1st-5th gears
and 6th gear taller than the original 5th by about 10%.
The results are excellent for most needs. Acceleration is
quicker without having to abuse the clutch, and cruising rpm
is noticeably reduced. 80mph in 6th gear is 3,000rpm...
very relaxed, yet the 3.8l motor has enough punch to
accelerate comfortably without downshifting. Top speed
is a theoretical 187mph, certainly more than prudently fast
on a public road.
However, note that I did change to the 3.64 gearset. The complete speed in gear chart can be seen by clicking here.
The details of this conversion do not end with the
transmission and gearset... Brett also installed a Quaife
automatic torque-biasing differential.
Click here for the Quaife explanation.
The coversion work was done by Brett Anderson of KoalaMotosport
(web: http://www.koalamotorsport.com
e-mail: brett@koalamotorsport.com)
and was absolutely flawless in execution. This
project, despite how straightforward it seems, was something
that I would not have trusted to anybody else.
