Hi Scott,

You've asked an important question, regarding fuel economy, and your truck's apparent lack of it.

First, all Duramax equipped 2500HD/3500 series trucks come with 3.73 differential gearing. The new 6-speed Allison with its double-overdrive will produce engine rpms roughly equivalent to a 5-speed Allison equipped truck with 3.42 gears. I assume you're running in 6th gear while on the Interstate.

The thread we began a few weeks ago describing a Scatter Plot would be a good place to begin when troubleshooting a low mpg truck. This requires an EGT gauge, and it might be worth installing as a first step (along with a boost pressure gauge). (Read the whole scatter plot thread - to get most out of it.)

We have a pretty good understanding of the relationship between EGT, boost pressure and fuel economy. It requires burning a certain amount of fuel to achieve a given speed. Exceptionally low mpg's must mean either your truck is producing higher than normal drag or the engine/drivetrain isn't running efficiently. Again, an EGT/Boost gauge would show that.

For example, a low boost pressure (due to some turbo or turbo control issue) would cause higher than normal EGT's and allow the truck to produce more smoke. You'll see this immediately with gauges.

IMO - The 2006 LBZ ought to be 20 or better when driven unloaded at 65-mph.

Jim