Series-hybrids can be fitted with a supercapacitor or a flywheel to store regenerative braking energy, which can improve efficiency by recovering energy otherwise lost as heat through the braking system. Because a series-hybrid has no mechanical link between the ICE and the wheels, the engine can run at a constant and efficient rate regardless of vehicle speed, achieving higher efficiency (37%, rather than the ICE average of 20%) and at low or mixed speeds this could result in ~50% increase in overall efficiency (19% vs 29%).