I had a 2004 Ram 2500 with 5.9 Cummins and auto transmission. 3.73 gears. Stock tires. 21mpg generally and never less than 16mpg. I never really towed heavy with it.Just had a thought, I do want to eliminate the clutch based limited slips
So blazer with th400, 3.08 gears can cram 20mpg.
I dont see why I couldnt at least match that with the overdrive and closer gears in a m1008. Especially since a blazer with th400 doesnt have lockup, so theres some parasitic loss at cruise and acceleration.
I think I remember people commenting on the 6.2 being able to pull the truck down the road at sub-2000rpm range upwards of 65mph. Although probably not very efficiently. But still the engine seems to have plenty of torque to be efficient at, say, 55mph.
I plan to drive pretty lazily. After all, driving habits are the biggest factor in MPG.
As to your jeep example, I don't think a gasser is a great example of what mpg is possible in a truck like platform. A friend of mine has a ram 2500 cummins and manual and gets 25 mpg at 55mph. Of course it has all the torque it needs to push it down the road and up mountain passes in OD, but push it over 65mph and suddenly it can drop below 10mpg. And those trucks I believe have 3.73 with option for 4.10. To match their torque, they also weigh a lot more than a m1008, but pretty similar aero.
I currently have a 2008 ram 3500 with 6.7 Cummins and G56 6spd manual transmission. 4.10 gears. 34" tires and a couple inches of lift. 18mpg cruising empty. As low as 12mpg towing a gooseneck at 36k.
The above isn't really relevant to the 6.2. Modern common rail compared to mid 80s tech.
I had an M1009. Got 20ish mpg out of it.
I get 14mpg out of my M998. KS to GA and back. Towing a welder dropped that to 12ish.
I still think you won't do much better than a stock M1009 for mpg with the 6.2.
I'm not sure why we're focused on mpg anyway. If mpg is the goal, these aren't the means....