A simulation tool has been developed that can be used to estimate a fuel economy while driving in a mode test of a motorcycle equipped with a continuously variable transmission (CVT) at an early stage of development. For a precise estimation of a mode fuel economy, it is necessary to accurately estimate the CVT ratio, the engine speed, and the crankshaft torque during driving in a mode. To achieve this, this study has generalized the transmission efficiency of a CVT system. This study has also derived developed balance equations that can take into account the transmission efficiency of CVT and the slippage that occurs when the centrifugal clutch is about to be engaged. In the proposed method, the pulley ratio of CVT, the engine speed, and the torque at the crankshaft were obtained first by solving the developed balance equations at discrete times during driving in a mode. Then, the results were checked against a predefined fuel consumption map, which was based on actual measurements, to obtain the fuel consumption at discrete times. Lastly, the mode fuel economy was calculated through integration over time of the fuel consumption data. Comparison of the calculated result with the actual measurement showed that the difference between the two was within 3.5%. This method has been found useful as it enables accurate estimation of the fuel economy of CVT-equipped motorcycles at an early stage of development.