Running down the interstate at 75+ mph unloaded I see about 20 mpg and about 21-22 mpg at 65 mph.

What are the 05s' getting? at 75 mph and 65 mph?

I am ordering an 06' and wonder what I'll be giving up.

 

That's pretty dang good out of an HO. I only got 17 @75. My '05 is getting just over 18 @ 75. I have < 1K miles on it...mpg seems better right out of the box.

 

Computer says this AM 19.4 mpg @ 70 mph.

Looks like I'll lose about 1.5-3 mpg average?

 

I just did a 150 mile roundtrip and was averaging 24.9 on the flats at about 65-70 mph and right around 20 going aver Donner summit ( about 8000 ft.) at about 70 This was according to the computer and I do believe it to be accurate as I used very little fuel. I have about 6000 miles on the engine now.
Around town and my country dirt road I am averaging about 14-16 mpg. I am using winterized fuel and am looking forward to see what striaght #2 will do as far as mileage increase. The truck also seems to get better mileage when it is warmer out.

 

Very hard to say. There are far too many variables. All I know is my '05 is about 1 mpg better than my '04HO. Maybe you can get 1mpg better out of an 05 or 06? Seems like the 600s (or 610s) suffered from some fuel/programing problems the first six months (Dec 03- May04). Reflashes have helped some and not others. I was worried that this new rig would get worse economy than my '04HO (555), but that is not the case. I have more power, quieter engine, and better MPG. Looks like Cummins and DCX worked out the 600/610 bugs...

 

Thanks guys. My preconceptions were wrong. I was prepared to give up a little mpg for the added torque, over my 555.