A hybrid flywheel powertrain system for a bus rapid transit (BRT) is designed in this paper, using a technical-economic study. The modern high-speed flywheel, which is used in the modern heavy vehicle hybrid powertrain, has high efficiency, low weight, and low initial cost. In contrary to the other powertrain hybridisation options, the energy conversion does not need using the flywheel energy storage system. Therefore, the power charge and discharge process are high-speed and efficient. The economic study is defined based on component price and fuel consumption cost. The model of the conventional and the hybrid bus is generated in AVL CRUISE software. Results show that the hybrid flywheel bus has better efficiency in BRT driving cycle in comparison with the normal Tehran City bus driving cycle. The optimal flywheel capacity is 1.4 MJ to use in Tehran BRT, which has ten months payback period. The reduction in fuel consumption is 28.61%.