VW 1.9L TDI
MB 1.8L 271
Same era engines, the consumption diesel ranges from ~224-358g/kWh equivalent over the same range where the gasser goes from ~250-450g/kWh. Diesels are less sensitive to changes in load in terms of efficiency, so making the vehicle more efficient won't make the engine run as inefficiently as doing the same thing to a gasser would. That being said, taller gearing and aero improvement help both significantly assuming the driver cruises at a lower speed of ~60mph.