Rebuilding the tacho to an electronic more accurate instrument
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.
Just tilt it. 
