A relativistic ray tracing code is used in conjunction with a Fokker-Planck code to calculate the dependence of current drive efficiency on plasma parameters and wave characteristics for top launch. Physical considerations include the interplay of wave polarisation, relativistic cyclotron resonance, optical path length, and electron collisionality. The current drive efficiency at a given temperature is a function of only two parameters: electron number density, and the parameter beta identical to ( omega - Omega 0)/k/sub /// nu B, which includes information on magnetic field strength, wave launch angle and plasma temperature. Peak current drive efficiencies can exceed by 25% the values for comparable inside launch configurations.