For the widespread application of thermoelectric generators, it is of vital importance to have convenient simulation tools in order to test the behavioral consequences of a thermoelectric generator in almost real conditions. The simulation by numerical methods of the performance of automotive thermoelectric generators (ATEG) allows for time- and cost-saving assessment of material combinations and variations of crucial design parameters. However, even in the case of promising simulation results, it is complicated to guarantee the ATEG capacity for reducing the vehicle's fuel consumption. This work presents a method to assess the fuel economy of an ATEG design. The procedure, which takes into account the ATEG power generation, backpressure, weight and the coolant pumping power, reveals that the maximum fuel economy value does not occur with the maximum ATEG power generation point. The method applied to the ATEG presented predicts a maximum fuel economy value of 0.18%.