I toyed with 40psi, and 38psi initially, but now run 43psi in my 275/70/17's and have been very pleased with mileage/wear/noise, personally. To me, 38 was way too spongy, and 43 seems to be my sweet spot for efficiency's sake.

Last fill up I topped 18.1mpg of about 80% hwy/20% city for that tank. Might've had a slight tailwind on the highway as I usually see high 17.X's mpg on the same stretch, but could've done better with cruise instead of aggressive passing that day ;)

When off road if I air down at all it is no lower than 36psi, but even then that is IF I air down. Most times I don't and have no problem driving in/out of ranch roads, river roads, and muddy surfaces.