I run 18 all the way around on the street (20 in back if I'm towing my backhoe), drop it to 12 front/16 rear for off road, usually the bed has at least 200 lbs of equipment (chain saw, fuel, oil, peavy, choker chain, block & tackle, tool box with a small floor jack & assorted tools etc.
I am almost always at the limits of capacity or overloaded