3.8S MOD speedo cable change

I think it is time to replace my speedometer cable. Speedo is relatively steady at speeds over 30 MPH but between 0 and 20 MPH the needle jumps considerably. I would like to simply pull the inner cable out and have a duplicate made up by Nissonger Instruments (USA Smiths agents) or some other speedo service shop. I am uncertain on the exact length so using the existing one as a pattern is best. I assume it is possible to pull the inner cable out from the speedometer end, and have it duplicated. Assuming I can get my hands behind the instrument this is my plan.

John, the problem is just as likely to be the outer cable as the inner, probably best to just pull the whole lot?