We just bought a completely stock 6.0. Filled it up 4 times so far.
The day we bought it we drove from TX to MO, on the way to MO we got 17.8mpg. I would say average speed of 70mph
The way home from MO (which I think was winter blend up there) we got 16.8mpg. same average about 70mph.
The next fill up was pretty much all back road driving going 9 miles to and from work. We got 16.1 mpg. most speeds around 45-50.
The last time I filled up was about half back road driving and the other half city highway miles. Going from the house of various shopping places (we live out in the country so it takes some highway time to get anywhere to shop, it is getting close to Christmas) and we got 16.7 mpg. most speeds around 65.

I am saving up to do the builtproof EGR and Oil coolers.
Troy