You will get an
accurate showing tacho.
The Spider 3rd series used an
electronic tacho. I did a conversion on another Alfa
years ago and here's what we did. Get a regular
electronic tacho from your auto shop. Then test it
with the cables to see how it works. This will save
time later. Tere are photos of the instruments,
we did the conversion on a Berlina, but the operation
is the same on a Spider. Veglia or Jaeger, both
will do. On an early spider the conversion will
do too, you'll get a better and more precise
instrument which is quite nice. Oh, nobody can see
what's inside.
The tacho to the right is a
Jaeger from a spider. The new electronic tacho we
used was this, there are similar standard in any auto
shop.
Remember to tilt the
instrument as the needle starts at five o'clock on the
old tacho.
Just
tilt it. 
Remove the glass etc so you
can remove the components inside. Then, using a
pliar to hold the shaft holding the needle in place,
remove the needle carefully from the new tacho. If the
needle is glued in place, try to heat it a bit. I
used two plastic bits to remove the needle, held them
under the pliar. Be careful as the needle is very
brittle.
Here we have the entire thing
when the needle has been removed. This is what you
will put inside your old tacho. Thew yellow arrow
points at the shaft where the needle was.
Then you fix the new
electronic tacho unit inside the old housing.
Cut off the rear half of the
old instrument The alloy thing is there to keep the
new instrument in place
Here we have it all together
as a unit. The needle is fixed and we are ready to go.
We kept the original background so nobody will know
what's inside. On my tacho the needle will rest at 0,
the old was at 500 revs. The electronic is far more
accurate too, which is good.
To secure the new instrument
we made a support so it does not fell off. The wires
are also held in place so they don't mess up when you
put the tacho back in place on the dash.
Ready to go then, good luck
