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 great.
