I have read alot of posts on different sites discussing gas mileage, and I have to say that with my own personal experience, there are too many factors going on that can dictate what you are likely to see (wind, grade, location, tire size, type of fuel, age of vehicle) and the factors are not constant. The best mileage I have ever had came on a trip to Missouri from Austin. I got almost 22 mpg both going and coming back. That has always been the marker for me to shoot for when doing road trips. However, on a recent trip to AZ, heading west with very gusty winds, I watched as my avg came in at 15 mpg and I barely made it into the next town without running out.
On that trip, my first tank, came in at 20.5, but then the driving conditions changed and you can see what happened.
However, on the return trip, I consistently hit 20 mpg, but I did have the wind at my back.
Two final points to consider: My truck now has one size bigger tires on it (265/75) so I have to factor that into what my actual mileage is. If I am trying to go 80 the whole way, my mileage will take a hit. If I am ok with going 72 or if its a stretch of hwy where the max is 65 and I keep it at 65, I am likely to see 22 mpg.