Okay Guys! Let's stop comparing oranges to apples here!

The Cummins under discussion is in a truck that weighs more than half again as much as a 123 MB. This truck also has probably AT LEAST half again as much frontal area with a MUCH worse aerodynamic coefficient.

Fuel mileage is determined MUCH more by weight and aerodynamics than it is by engine size.

When I bought my '91 one ton, flat bed dually, 5 speed Dodge Cummins, I came out of a one ton dually, flatbed Ford 460 Four Speed. Pulling 15,000 pounds with the Ford I got LESS THAN 5MPG. Pulling the SAME 15,000 pounds with the Dodge took me to 14MPG. I thought my first fuel bill was a mistake.

Now, go figure ton/miles per gallon of a 3,500 pound 123 at say 30MPG and then go figure ton/miles per gallon of a 20,000 pound gross weight Dodge and gooseneck trailer at 14MPG and see which one is more impressive.

It is interesting to discuss the venerable Cummins and the venerable MB 616/617 in the same thread, but DON'T get carried away with the comparison of oranges and apples.