A virtual evaluation method that computational fluid dynamics combines with a mathematical model was proposed to compute the temperature distribution and final power output of an onboard automotive thermoelectric generator (ATEG), which was configured for an off-road vehicle called Mengshi. A road test was conducted to validate the accuracy and reliability of the virtual evaluation method. The output power and voltage of the ATEG in the road test showed that the virtual evaluation method exhibited satisfactory accuracy in predicting the performance of the ATEG and could provide effective guidance for the design of the ATEG. Furthermore, dimpled surfaces were introduced to replace the inserted fins in the conventional hot heat exchanger to reduce the pressure drop but maintain the temperature difference between the hot and cold end of the ATEG. Computational results showed that, compared with the ATEG with inserted fins, the pressure drop in the ATEG with dimpled surface was reduced by 20.57%, and the net power output was increased by 173.60%.