I've always experienced poorer mileage in the winter months...currently getting about 210-220 miles per tank.(until the light comes on)
I always let it warm up for about five minutes before I roll, so that doesn't help.
I'm currently running 45-50 psi in my Terra Grapplers, & run about 60 psi for highway use in my Mud Grapplers. I like to go on the high side for less rolling resistance, but admit that's a little higher than most would use.