Roadkill did a great job in his post. I will only add that you need to look at each set of tires, and check what the max rating is, what ply they are, what loads you are carrying, what roads you are driving, and several other factors. We've all heard about the Firestone tires and Ford Explorers, that was about the worst case scenarion of under-inflated tires and poor performance I know of. The tire may be fine ,but too little pressure can destroy it. The pressure may be within limits, but the tire load rating may be such that you shouldn't even use it on the Bronco. As a rule of thumb, I run at least a 6-ply tire, or 8-ply if carrying any cargo or towing. Add tire pressure for cargo or towing, up to the max, be it 50 or 60 psi. You can adjust the pressure up or down to improve ride quality, but generally not more than 5-10 psi. Start with 40 psi, and work from there, for most 6-/8-ply offroad tires. I have 10-ply Firestone Destination MT's on my '06 Dodge Hemi, and the tire shop is always trying to put 80 psi in them (the max reading). At that setting, I can't hold the truck on the road because the tires bounce all over the road. I run 45-47 psi on the highway. When I go offroad, I don't change the pressure at all. While it's true that a lower pressure of 20-25psi will give you more traction, you risk popping the bead off of the rim the lower you go. THen you have to air bakc up when you get on the road again!!! To get better traction in sand or loose dirt, you can prolly safely air down to 15-20 psi. WHen i go rock crawling with the Bronco, we run 8-10 psi, but I'm running on Hummer Hi wheels with the internal bead lock. Running that low a pressure on a 16.5" wheel is a sure bet to pop the tire off, since that size has no safety bead like smaller or larger diameter tires. JSM84