Volvo's been using overdrive transmissions for a very long time.

The first manual transmission that I know of, the M30, had an

overdrive option that allowed overdrive in 2nd and 3rd. These

transmissions were mounted to the B4, B14, and B16 motors. When

volvo went to the B18 motor, they also introduced a true 4sp

manual, the M40. This later in the 1800 had an overdrive options

(that was also an option on the 122 and later the 140/240 cars with

the B20 motors). My impression is that the M45 is almost identical

to the M40, but for some minor differences like being setup to mount

to a slanted motor and so on. Thus, it would have been trivial for

volvo to continue to use the same design for the M46, the overdrive

version of the M45.

The problems with the overdrive setup are:

needs electronics, hydraulics, and mechanics to work correctly

in order to work.

weighs more than a normal transmission, takes more space under

the car

has unfortunate failure modes (reverse + OD=boom).

Otherwise they seem pretty good.

chris