Traction efficiency of farm tractors barely reaches 50% in field operations (Renius etal., 1985). On the other hand, modern trends in agriculture show growth of the global tractor markets and at the same time increased demands for greenhouse gas emission reduction as well as energy efficiency due to increasing fuel costs. Engine power of farm tractors is growing at 1.8kW per year reaching today about 500kW for the highest traction class machines. The problem of effective use of energy has become crucial. Existing slip control approaches for farm tractors do not fulfil this requirement due to fixed reference set-point. This paper suggests an optimal control scheme which extends a conventional slip controller with set-point optimisation based on assessment of soil conditions, namely, wheel-ground parameter estimation. The optimisation considers the traction efficiency and net traction ratio and adaptively adjusts the set-point under changing soil conditions. The proposed methodology can be mainly implemented in farm tractors equipped with hydraulic or electrical infinitely variable transmissions (IVT) with use of the drive torque feedback.