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One of the 3 main reasons that diesels get better MPG is the lack of a throttle = almost no vacuum, lots less pumping losses.

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Probably a silly thought, but what about using turbo(s) to provide 1 or 2psi while cruising down the highway; just enough to overcome any pumping losses?

I'm not sure if that would provide more gains than the cost of the extra fuel needed and loss from backpressure on the turbo(s) though.