What off-road use tire size for 18" rims?
 

The new truck has 18"/7.5" GM accessory rims and 265/65 tires. Is there a decent off-road tire size upgrade for light duty off-road use for this rim or should I just retrofit 16" rims on the truck and go back to 285 duratracs again?

Not sure what you are asking...Do you want a bigger sized, off-road tire?

If so, try Nitto Trail Grapplers, I run them (295/75/18). And aside from them being Japanese, I love them. They look good and perform well off-road.

I also run the same size tire in the trail grapplers. they work great. if you are gonna offroad much at all I wouldn't go any smaller. Your other option is 35/ 12.5/18s.

Going by the old rule of thumb for tire height, it should be twice rim diameter. 35's would be much better for airing down offroad.