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One thing that really helps MPGs on autos is EGR. With many under cruising, up to 40-50% of intake is exhaust. This reduces pumping-losses due to intake-manifold vacuum (less difference between pre-throttle vs. post-throttle pressures). Pistons don't have to fight mostly-closed throttle on their intake-stroke. Power to generate this vacuum comes from other pistons on power-stroke.
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