1977 f250 speedo issues
 

replaced the speedo cable on my buddys 77 f250 as the old one was cut.i swapped over the gear from the old one and got it hooked up. at the cluster i just pushed it in as not sure if theres a trick to it or anything.

anyways the speedo is about 10mph off. when sittin it says 10mph and when accelerating it surges all over then at a steady speed about 10mph off.

what did i do wrong? the cable is from LMC