Well just to put my 2 cents in here... The 33'" tires aren't helping the mpg (heavier tire and more rolling resistance) My first thought was you might haven't correct your speedo with the turner...but I see that you did. Wind also plays a factor. But two of the biggest things is 5600 lb truck vs hills equal bad gas mileage, and anytime you get over about 65 in these trucks throw the gas mileage out the window.
Here is a little data I gather about my truck on a 280 mile round trip. Most of this is interstate. I found that if I put cruise on 67 mph (65 according to the gps) I can get around 18.9 Droping down a little more will get me 19,4+
I also found if I put the cruise on at 77(gps=75 actual speed) I drop down to around 15.5-15.9 Sometimes a little lower but thats about the average. Now before I get slammed for anything I said the conditions was pretty much the same on winter blend fuel running canned level 2 3.73 rear end. Also mpg was being calculated by miles driven divided by gallon's of gas, not programmer.
But just to throw this out there if I run around 55 pulling the ole bass boat I can get around 17.5 then the mileage drops after that. But with the tire upgrade coming those figures will be off now
