Duramax vs Power Stroke Fuel Economy Tow Test and Drag Race

ShinerBock wrote:

4x4ord wrote:
So I guess by locking out 6th and running 1800 rpm you gain a little responsiveness and have less gear hunting but it's going to cost something on fuel.

Not exactly. I have tested it out multiple times towing my RV 160 miles down to the coast and other trailer like my cattle trailer. I did it 6th a few times and then in 5th. I averaged about 1-2 mpg better towing in 5th than I did 6th. I noticed on my gauges that the engine did not have to add as much fuel/air to make torque/power in 5th like it had to in 6th when going up hills.

We also noticed the same when I worked for Cummins. The "sweet spot" for the engines best efficiency was not static and changed depending on load. Unloaded it was between 1,300 and 1,500 rpm and loaded it was between 1,800 and 2,100 rpm depending on load. This is why Cummins' PowerSpec application recommends these rpms when loaded for best efficiency out of the Cummins.

Cummins PowerSpec

It would be interesting to see the brake specific fuel consumption graphs for these engines. You'd expect the transmissions to shift according to fuel usage.At 1800 rpm these engines can produce over double the power that is needed to tow a typical 5ver down the highway. I'm surprised lower rpm wouldn't save a little fuel. A general rule is that higher cylinder pressure produces greater efficiency. If you're running higher rpm than is needed the engine is operating at a lower cylinder pressure than it could be.