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Hybrid technology is being used in some commercial trucking applications already. For example hybrid garbage trucks are starting to get purchased by many cities, where they make sense due to the gains made by regenerative braking. Some local delivery trucks are also using hybrid technology. Unfortunately most tractor trailers are intended for long haul (1000+ miles), where adding complexity and weight of a hybrid doesn't make sense and would decrease mileage, and/or increase maintenance cost. The easiest solution to improve gas-mileage of a tractor-trailer would be to redesign them to be more aerodynamic. The could put a shroud between the cab and the trailer. They could put ground effects around the tires. The could put movable panels (which could be designed not to interfere with docks) to smooth the air flow off the back of the trailer. They could design windshields of the trucks to be angled like in modern cars.
Though the general population resists style changes, if a manufacturer could find a large trucking company and get them to replace their fleet with aerodynamic trucks the rest of the industry would slowly follow suit.
BTW a tractor-trailer has a very low MPG somewhere near 6 MPG