Chase the torque, not the power.

In terms of the 5th gear thing, it's because diesels run significantly
longer gearing than petrols.

Good grief..*way* outside how you *should* be driving it!

Why do you have a manual car and not want to change gear?

Correct..it isn't designed for that.

Ouch..I can feel the engine hurting at this low-rev torture.

You should fix your lazy and unsafe driving technique. I was taught,
for safety, to always be in a gear that allows the car to accelerate
immediately and rapidly by just depressing the accelerator without the
need for a change down.

Diesels, these days, develop their maximum torque,typically, about
1700-1800rpm which is *much* lower than a non-turbo petrol. If you try
anything much below that (say 1200rpm), you will be off-turbo so the
engine will be significantly weaker. However, even at very very low
revs, the torquey diesel is magnificently resistant to stalling -
sometimes to the point of being violently jerky. It will shout and
scream at 5th gear 20mph attempts as that torque is being dissipated
in the engine bearings.

In *ordinary* (i.e. not blasting about) use, the only difference
between a diesel and a petrol is (in broad terms) that the diesel
works between 1800 and 3800rpm whereas a petrol will work in
2500-3500rpm or slightly higher in each case. I've always maintained
that, due to gearing ratios, the apparent power band width of petrol
and diesel cars is similar. For example, the difference in revs
between gears in a petrol car can be 1500-2000rpm. Also, the diesel
will not rev as highly as a high-revving petrol.

As for why your car does 20mph pull-aways in 5th gear, I suspect it
has so little torque at those revs that it simply hasn't got the
output to make any complaints obvious but I wouldn't want to drive any
car like that.

This is the complete opposite of most diesel vs petrol views especially
with Alfas!

Alfa petrols are traditionally high revving engines requiring lots of
gear changing to enjoy. The modern TS engines have a bit more torque low
down but nothing to compare with a modern turbo diesel.

The general view from petrol heads like me is that Italian cars are
*meant* to rev, hence I find diesels at odds with that. The alternative
view is that times have changed and high torque diesels are just as
quick and a more relaxed drive with economy. It comes down to preference.

As Zathras says you must be off turbo in the diesel and as Steve says
the gearing is different as diesels have a lower and more limited rev
range so the gearing is higher.

Try accelerating hard in 5th in the diesel at say 40mph and you would
need to change down a couple of gears in a petrol to achieve the same.

The issue most have going from a sporty petrol to a sporty diesel is the
opposite, they will find they run out of steam just when the petrol is
starting to fly because they are in the wrong rev band. Which is why it
doesn't always feel sporty as less is more with the diesel.

Cheers
Tony