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Or are there other factors that I am not aware of. Thanks.



Compression ratio and cam profile.

The volumetric efficiency point of an engine exactly matches the torque peak. If this torque peak occurs lower enough in the power band and you have a high enough compression ratio to squeeze it without detonation and this new higher rpm is closer to the torque peak, your mileage may go up.

eg; my 74 D200 gets better mileage at 75mph than at 55 mph. At 75 mph the cruise rpm is closer to the torque peak and it produces more useable power with less throttle, so the engine is working more efficiently.