Like Cobra said it is 06 LLY followed by the LBZ's. I think Wendy is a little more thirsty then Casper and Blue. But I have noticed since I hit 9k it's getting a little better. So maybe she's finally breaking in
But LBZ run lower compression so that might be the difference. But it's a good thing I'm not driving Duramax's for MPG
I think any mileage differences are on the highway. IIRC, the first five gears on the 6 speed are the same ratios as the 5 speed Ally. It's the sixth gear that is a little lower, keeping the rpms down on the highway.