Speedometer cable for 1965 Plymouth Barracuda 273

Ive used them. My first one I cut it too long and the thing was binding up in the sheath and making the speedo wiggle. The 2nd one I cut just a har shorter and it has been working ever since. Use graphite powder (lock lube?) as the lube not grease or oil as that collects garbage. I was suprised how gritty the old cable was when I pulled it it, where did all that stuff come from?
Would it be best to cut to same length as old one?