The best off-road performance modification you can make to your Jeep is free. Yet nobody seems to do it, or if they do, it's almost always done incorrectly. I, too, was once a nonbeliever. Early on when I went wheeling, I refused to air down my tires. I didn't have an onboard air source, and I was terrified of driving on the street with aired-down tires. Plus, I figured I would cause unnecessary wear to my brand-new, show-stopping 33x14.50 Mickey Thompsons on 15x12 wheels.
Needless to say, I got stuck a lot. Of course, I filled the tires to the max-rated sidewall pressure. Having 30 psi in an overly wide, bias-belted tire translated into nonfunctional contact patches off-road. I thought the wider tire and wheel combo would negate the need to air down. I even argued with a buddy about it after having been stuck on the side of a sand hill, crying and begging like a little girl for him to back my truck down for me. I was wrong. Months later, my expensive tires wore out anyway, but not on the edges as I had feared airing down would cause. They wore out in the middle due to over-inflation instead.
Today, if I go off-road there are very few tires I would run at more than 10-12 psi. I wouldn't recommend it, but I once ran a set of 38x15.50s at less than 10 psi the entire time I had them. I had 10,000 hard miles on- and off-road on 'em with a rear spool, and they still had about half the tread left when I got rid of them.
Not all my Jeeps have bead locks, but If I'm hitting the dirt with them, I'll cut the air down to 5 psi and sometimes less. The traction you gain by airing down to 2-5 psi with large bias-ply tires is unmatched. My 49-inch tires were worthless off-road at pressures as low as 5-10 psi, but when I bumped them down to 2 psi, they grabbed like Mexican kids at a sticker toss in Baja.
Every tire and every Jeep is different. The correct pressure for your Jeep may be different than your buddy's rig. Air down until you get a good sidewall bulge. In soft terrains like sand and mud, you can go a little lower than in areas with rocks that can cause tire damage. Try different pressures until you figure out what works on your Jeep.
Thousands of dollars have been spent on fancy bead locks and onboard air systems only to be used ineffectively. It's almost embarrassing when I see guys with fully decked-out rigs struggling on obstacles that a properly aired-down stock Jeep can pretty much walk right up. Nowhere is this more obvious than in the sand. By the looks of it, many of them seem to run tires at pressures in the 15-20-psi realm. That's street pressure for most Wranglers, in my mind. You don't need bead locks at these pressures, so don't waste your money if that's your plan of attack. Well, unless you simply like the look of them and don't care about how your Jeep performs. Ultimately, I'll be the one who's driving the properly aired pile up the obstacle Mr. Big Pump had to winch.
--John Cappa