A Mileage Melee


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.

Intercooled or not, stock or modified, electronic or mechanical injection, diesels have one inherent fuel economy advantage that’s often overlooked: No throttle! Yes, diesels are always at WOT, output is adjusted via varying the rate of injected fuel. The result: Way less pumping losses, and better mileage, especially at light loads. By the way, while Rudy Diesel may have invented the compression-ignition engine (and even that “fact” is disputed), it was indisputably Clessie Cummins who made diesels work in any application with varying loads and/or RPM requirements (cars, trucks, boats, etc.).
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