A split-axle parallel hybrid drivetrain with in- wheel motors allows for existing fuel-powered vehicles to be converted into a hybrid electric vehicle (HEV) with minor mechanical modification, to result in a retrofit-conversion HEV. This is achieved by placing electric motors in the hubs of the otherwise non-driven wheels. Due to the small design space of the wheel hub, the size and power of the in-wheel motor that can be installed is severely restricted to less than 10kW per wheel, raising the concern of actual performance in improvement in the converted hybrid vehicle. This work analyzes the impact of motor size and efficiency on fuel economy of three different converted hybrid vehicles, through simulation using rule-based control strategy. Results provide insight into the sensitivity of different-sized vehicles to the size and efficiency of the retrofitted electric motor. © 2013 IEEE.