mr.singh, I did tend to agree with your thoughts at one time
as to why they reverted to 5 gears in the 451 over the 6 of the original 450,
but then I found that when I dove Rob of S2's own 450 CDi it ran out of
'pull' at around 70 but 'pulled' thru it if I changed down to 5th.
So, one can see why the change of thought and development at Smart
brought about the 5 gears in the later 451. Torque suited it better too,
and let's face it...it wasn't designed to be a motorway cruiser, it was
designed as a Town Car.
As for the 454 argument over the 5 speed manual versus the 6 speed
AMT.....I have both, and I have to admit that if the AMT had instead
been a 5 speed then there would be quite a noticeable step in ratios
when going up or down through the gears as it does not have the 'luxury'
of fluid dampening flywheel.
As the tranmission was shared with Mitsubishi I feel sure that both
compaines looked into every aspect of 5 versus 6 speeds for the AMT
on the Colt/ForFour issue and for whatever reasons the 6 speed won.
As for it being a tad more economical than the manual....just lucky
with the way it turned out I am inclined to think.