All of this back and forth on MPGs is pointless.
I'd bet nobody is hand calculcating and avergeing their MPG for every single fill-up. I'd suspect most are posting their "hero" tank. All highway driving, perfect weather, etc...
Speaking of weather- a vehicle in, say South Carolina, will get *way* better mileage than one in Minnesota. Cold starts waste a ton of fuel. A cold drive train drags down your average until it heats up.
Then you get into actual differences in driving styles. I can easily lop off 25% of my MPGs in every vechicle I own by simply driving like most morons around here. Accelerating towards red lights. Mashing the gas from every stop, gotta get to that next red light fast. Hitting the brakes on the highway instead of anticipating what's happening in front of you and simply letting off the gas earlier. Coming to a full stop instead of slowing earlier and keeping it slowly rolling towards a red light- in the places and times when you can do that.
There are hundreds of ways to save fuel while driving, and just as many ways to waste it.
All that said- if everything else was completely equal, the added wight of the 37s may be offset by the reduction in final gearing due to the larger tire. However, the 10 speed makes even that that dubious. The truck is not begging for a higher OD gear like older trucks with less gears.
