I think you'll find this is caused by poor/dirty ground strap from the engine to the body inside the engine bay.

It only takes once, and the cable it screwed, as it melts the nice polyethylene coating on the inside.

See how the cable's protective plastic appear worn..I'd say it melted during starter crank. Then when the cable gets some speed up, the plastic grabs and the cable snaps.

Strange... The cable in my car is the original one I bought it with, which no doubt must have seen a few 100 thousand K's of mileage. I was still pondering the speedo setup when I realised that when I remove the W58 gearbox's electronic speed sensor, the Datsun cable screwed straight in there, it's drive even looked exactly the same as the electronic one... Have hit 0km/h a few times since the car got going, and though it does jump around sometimes, it's never broken...

Dad had a Ford once which also ate speedo cables at an alarming rate, there were no sharp bends in the cable/s anywhere. He eventually fixed it by putting some graphite powder in the outer along with the cable. Graphite powder is an excellent "lubricant" in such applications, because it's very slippery(duh), and it doesn't attract dust, which is a sure killer...

I wonder if the speedo itself might not be slightly "tigter" than normal...

Like is said, this is now the second one to break and the UTE now only has 110000km on the clock. Ill have a check for any sharp bends in the cable, but I doubt it thought.

I have never broken one in over a million miles of motoring, so there must be something wrong with the way it's installed. If there is a bend, or kink in the cable run, or excessive strain at the fitting, then you have problems.