Energy-demand modeling for the needs of energy management relies on two major approaches: (1) The econometric approach; and (2) the engineering, or technoeconomic, approach. The former emphasizes the explanation based on economic behavior and the estimation of the model by statistical methods; the latter is based on a detailed description of energy uses, stressing the importance of the energy efficiency of equipment and the quantification by means of technical and judgmental data. Despite the clear importance of these two approaches for the improvement in reliability and scope of energy-demand models, research aiming at a synthesis has been rare and timid, if not from a theoretical point of view at least as far as empirical applications are concerned. In this paper we present an econometric model built according to an engineering logic that takes into account the main technical and behavioral mechanisms influencing energy demand. Applied to the modeling of motor-fuel demand in Switzerland, using time series, this model allowed us to evaluate the effectiveness of the carbon tax proposed by the Swiss Federal Council in reducing car CO2 emissions.