setVelocity [0,5,0] will always set your tank's velocity at 5 m/s north. So whilst this will work if the tank is pointed directly north or south (since the velocity will be in the driection of the tank's tracks), it probably wont do anything much if the tank is pointed east or west, except give it a small shunt.
If you want the tank to always move forward, you'll need trig.
e.g.
setVelocity [5*sin(getDir _tank), 5*cos(getDir _tank), 0]