I've to ends of my speedometer cable but no middle is this possible? It goes behind the dash gauge cluster and comes out at the gesr box but I can't find the Fire wall gasket, I have been under, over and through but can't see it. Any help is welcome....Mike

The speedo cable comes out at the top of the transmission tunnel. If you get your head in the drivers footwell and look behind the heater box you'll see it. Also if you have small hands (use your wife or child), under the car, run your hand along the cable. It will lead up and over the gearbox.

Some models have a junction box against the inside firewall. The cable from the speedo connects to one end and the cable from the gearbox to the other. This is a little gearbox connected to a trigger. I believe that it serves as a pseudo service switch as it has a reset function.

I just swapped mine ('79 carb CS2) a few days back.
I followed the route of the old cable which was through a hole just below the brake fluid reservoir on the firewall almost directly behind the speedo.
There was a grommet/boot already on the cable to keep it nice and snug in the hole.
It then disappears down the left hand side of the engine and back to the gearbox.
Its exactly the right length going this route, without kinks or fowling anything on its way so I guess its correct.

This is yet another one of the things Fiat monkeyed with over the years, and for no good reason that I can find. Some go through the firewall with all the rest of the cables and electrical, and then others go through the transmission tunnel. Just take it loose at the ends and follow it.

Speedo cable is inj and working,kind of, the speedo cable had a huge kink by the tranny tunnel, but the speedo itself might need an overhaul