This paper presents the results of research, whose aim was to design and build a prototype device for the estimation of traction parameters of a car, including power output from engine. The estimation of the power of the powertrain system follows on the basis of measurements of the car's longitudinal acceleration in a special road test in actual driving conditions. The measurement of the longitudinal acceleration brings a number of problems which were solved due to the application of the new measurement method. The factors affecting deficiency of the precision of measurements and the methods applicable for its reduction were developed. As a result, it was possible to obtain a measurement method with reduced uncertainty and increased repeatability in terms of results. In addition, an interpretation of the results of the test is presented in reference to the power of the powertrain and engine.