All my other gauges work fine, but my speedometer gauge doesn't? Any ideas why? I'm hoping this is an easy fix & doesn't require any harness work?

 

Check out the cable, pull both ends off and look to see iif there is a bundled up wadded mess at either end, if so replace the cable. Also if the cable is ok then check the transmission gear to make sure it is not stripped. If all of that is ok then it is in the speedo head itself.