The drilling specific energy (SEdrill) is a very significant measure of drilling performance. It is directly compatible with cost/metre, because it relates to the amount of energy required to penetrate rock. SEdrill can also be used to quantify the efficiency of rock working processes and to indicate bit conditions, rock strength and rock hardness during drilling. The focus of this study is to evaluate the optimum performance of polycrystalline diamond compact (PDC) and tungsten carbide (WC) bits using a criterion based on maximum feed rate at minimum SEdrill or vice versa. A variety of coal-measure rock types were drilled using PDC and WC bits using a fully instrumented and automatic drilling rig at six different rotational speeds (rpm) and an over range of thrust applied to the bit. The operating parameters of the drill bits were monitored on a computer-controlled logging system during the drilling trials. The control system is used to monitor the drilling speed, the thrust, the power developed at the bit, the feed rate, the distance penetrated and the presence of an adequate amount of drilling fluid. The strategy followed while drilling was to maintain a constant level of power at the bit in order to maximise the feed rate and minimise the rate of bit wear. Extensive laboratory tests have been carried out on the drilled rocks including mineralogical, physical and index properties analysis. Similarities between the rock strength and SEdrill are reported. The destruction specific energy (SEdes) was estimated from the area (integral) under the stress-strain envelope in an unconfined compression test. This method provides a better understanding of the relationships between drilling specific energy, drilling rate and the main mechanical rock character. The relationship of the SEdrill to operating parameters of the bits and the rock properties was evaluated using optimum drilling data.