Mileage Melee

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A friend of mine bought a
used ’95 Ram pickup with
the 5.9L Cummins turbo diesel.
He’s getting almost 3 MPG
better than my new 2010 5.7L
Hemi Ram, even though mine
has the MDS [multi‑displacement
system that shuts down 4
cylinders on the highway—Ed.].
I say this is because the diesel
has a much higher compression
ratio. He says it is because
diesel fuel simply has more
energy per gallon. Who is right?

You both are—the increased
CR is significant, and so is the
fact that diesel has over 10%
more BTUs per gallon. You
have, however, both overlooked
a third, possibly more important
factor: Pumping losses. At light
throttle, your Hemi is pulling
lots of vacuum—the pistons are
working hard pulling that vacuum
against the almost‑closed
throttle. The diesel is always
at wide‑open throttle, so these
losses are greatly reduced.

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