Researchers at the University of Toledo are fine-tuning a hybrid hydraulic propulsion system for heavy vehicles, the Associated Press reports. The research could double the vehicles' fuel efficiency while cutting down on pollution, Walter Olson, a mechanical engineering professor leading the research, said in the AP report. The hybrid hydraulic propulsion system stores the energy generated by braking, which powers a motor that supplements the force of the vehicle’s engine. Heavy vehicles benefit the most from a hydraulic propulsion system. Researchers say the system would increase a typical four-wheel-drive SUV’s fuel economy from 17.2 miles per gallon to 34.6 miles per gallon. The vehicle's initial cost would increase by between $500 and $3,500; however, Olson says fuel savings would be about $5,000 or more over the vehicle’s lifetime.