Going from memory, I seem to recall the MB 4-cylinder diesel cars my Mom and Dad bought new in the 1960s through the 1970s, stated their engine compression as 22 to 1.

I was a dealership (not just MB) brochure collector and specifications reading youngster and teenager in the 1960s/1970s.

Higher compression ratio == higher BTUs/efficiency, right? If that's true, then that would contribute to better/higher mpg.

I am still thinking that the low RPM - high torque range of the 3.2 liter / I6 / CDI, COUPLED with a low numerical rear end gear ratio, may also account for the outstanding E320 I6 CDI mpg. The I6 propels the CDI 0-60 in the 7-second range - so these are not slow cars off the line.

The 2005 E320 CDI (5) of them as I recall, were run at the speed of 145 +/- mph in a relay type of sequence around the clock at the Goodyear Tire Company test track facility located near Laredo, Texas to complete some sort of miles run endurance record/challenge in 2005 or 2006, by Mercedes-Benz Germany - so these are cars unmodified except for their engine governors, that achieve high speed too!

Brian Carlton could probably look at the numbers/specifications and help identify the differences in why the 3.2 Liter I6 CDI sedans produce such good mpg numbers - versus the 3.0 V6 Bluetec sedans.......?