depends on what size cable. i ran my tach on flstc (3 wires) w/o removing tank. took a little time, but was simple. route it along with the factory wiring. good luck...
It might help if we knew where are you running it, how big is it,and what's it for?
Generally taking the tank offwould make things a lot easier. But its not absolutly necessary, especially if the wire is small.
I just ran a cable for my Garmin GPS under the tank. I used a piece of weed eater line and snaked it through then taped my cable to it and pulled it through.. Worked great..