Re: 6.0L Fuel Economy
My Aunt and Uncle have a 2003 2500HD with the 6.0L. On the highway they get 13mpg and around town 11mpg. Towing they get 10mpg if they don't have to deal with wind or mountains.
They're [bleep] stupid for not buying a truck with the 496. Still get the same gas mileage but you've got buckets more power.
I think it's just a side effect of moving a 7000lb truck around all the time. Really, the 6.0L doesn't have enough low-end gonads for a truck that weighs that much.