To add a line. High mileage is result of controlling the Thermal efficiency. Every diesel or petrol engine converts heat into Mechanical power. Enjines have many moving parts and energy is used to control. Absolutely there is losses. Due to such losses fuel economy is low. Losses such a fuel pumping, multiple pulleys and motors, exhaust, strikes eyc. Eventually controlling the thermal efficiency ( inputs and outputs) brings greater millage. Engineers have technology and tools are used to reduce and such are expensive