A number of mathematical models with the use of whom one can study the diesel engine performance have been proposed throughout the last years, aiming the optimization of the torque-engine speed dependence. The authors, having the experimental data for a D-115 engine, regard the fuel consumption equation as a function of torque and engine speed and use the equal slope theorem in order to obtain the equation, which indicates the optimum engine speed of the engine. Having this equation, one can develop a program requested for the design of a gear selection indicator. This will display the optimum gear selection for the actual engine speed and torque.