I usually get about 17.5 mostly highway in my RCSB 4.8 2WD with 3.73's. This is at 70-75 MPH. If I am going against a strong headwind, which happens quite a bit, it drops to about 16.5 MPG.

I took a trip up to dallas though a while back and averaged about 21 MPG on the way up and about 20 on the way back.

 

ive seen 18.5 on a 250 mile trip and have seen as low as 14.2 on the same trip and i get about 13 when I am just driving around town

 

5.3l, AFM, ECRB, 3.42's with just over 6k. I've been getting 18.5-19 in the city. I took a trip to GA with about 1500 miles on the clock a while back. I used cruise the whole way down and got 20.5 and back up without cruise I got 21.5.

 

I must have gotten lucky. My 6.0 will get over 20 all highway and averages 15+16 city highway. And I've got a medium sized cam installed.