How many 3.3 on Fuelly are work trucks though?
Associates 2.7 is around 15-16 MPG in fleet kit and duty.
Plus nobody posts axle ratio or other important info. Half the trucks I click on are just the default 50/50 city/highway pie chart.
I figure if there are enough data points, then the anecdotes pretty well average out. It's good for comparison purposes. If you want to guess what you'll get, then compare your current MPG to what they say for your rig, and use that same ratio for the one you are looking at. For instance, 17 mpg for your 3.7 vs 19 for the 3.3 is a 12% improvement. If you can think of a better way to guess then use it!