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Also, as for fuel efficiency, I'm going to assume your Duramax has a tachometer. I've always heard and found from personal experience that 2,000rpm is the magic number. Going higher from there and the economy drops off like a rock. Kind of like how drag increasing much more the higher you go. It isn't a straight line drop. Logarithmic increase in drag with speed. Seems the same on rpm to me.
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