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The Pressure: Where Should You Run Your Tires?

Posted in How To: Wheels Tires on January 22, 2018
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You all know by now that airing down your tires is one of the simplest ways to improve the trail performance and ride quality of your Jeep. The big question is always: To what pressure should I air down? Whenever this topic comes up on forums or social media, the answers vary from to “tire circumference minus rim diameter, multiplied by pi, minus GVWR, multiplied by ambient air temperature, and divided by your age” to “6.” What I have found is that there are two methods to effectively and safely adjust tire pressure that apply to a variety of vehicle and tire types.

The first and easiest way to adjust tire pressure is to start by simply dropping the tire pressure by 10 psi below the usual highway tire pressure, or what the placard on the doorjam says the tire pressure should be. On a Jeep, the placard psi is usually 32 to 37 psi, so dropping 10 psi is a 27-31 percent decrease in pressure.

Determining what your tire pressure should be depends on how much your Jeep weighs at each axle. Find a shop with individual wheel scales to get really interesting results. Note that the max psi states “cold.” This is because a warm tire will read more pressure than a cold tire. Tire pressure can vary by 1 psi per 10 degrees Fahrenheit.

Though I like to run at about 12-14 psi on most trails with my 6500-pound Jeep, not all tires and driving styles are appropriate for that. Even if you don’t know the ideal pressure for you and your rig’s tires yet, simply reducing the air pressure in your tires by just 10 psi will allow your tires to grip the trail noticeably better, reduce the likelihood of puncturing your tire on a sharp rock, and improve the ride quality for you, your passenger, and your Jeep’s components. You can always let a little more out to see if it helps if you find yourself constantly losing traction on the trail. Don’t think 4 psi can make a difference? Think of it this way: if you are at 20 psi and drop to 16 psi, you just increased your footprint, that is, your traction, by 20 percent.

The second method is geekier and more technically correct, but requires some math. Let me spell out the disclaimer here: This is the formula that I have found works for me with the variety of vehicles and tires that I have run. It is not meant to be the law for all tires and rigs out there, but rather a guideline as a reasonable starting place, and can only be accurate if you know the actual vehicle weight at each axle. No tire or vehicle manufacturer will endorse a formula for airing down. Do so at your own risk!

A nice, soft tire wraps around rocks and pulls you over them gently, rather than bouncing and spinning off of them. Modern sidewalls can handle even extreme twisting like this tire at 12 psi on a 6000-pound Jeep.

The sidewall of the tire will state a max load at a specific psi. The max load stated is not necessarily the street tire pressure you should be running. It is the maximum load that the tire can sustain. To determine the air pressure for the weight of your rig, run the following formula:

Weight at the axle divided by the max load stated on the tire multiplied by two (since there are two tires on each axle). Add 20 percent to that number for a safety margin. Now multiply that times the max load pressure stated on your tire. This number gives you the street pressure for your tire. Divide that by 3 for the lowest minimum off-road pressure for your rig on these tires.

Weight at axle
________________ = A
Max tire load x 2

A x 1.2 = B

B x Max load psi = C (your street pressure)

35% of C (or C x 0.35) = your lowest minimum off-road pressure

Clear as mud? When in doubt, always round up or add 10 percent for extra safety. If you are driving with any speed, you should add a little more pressure. This formula has worked well for me as a starting place for many of my rigs, with often only an adjustment of a couple psi here or there. I hope it helps you figure out your optimum tire pressure as well. If not, I would love for you to yell at my editor.

Remember, safety requires that you have a way to air-up before long trips home on the highway, especially when dropping below 20 psi. Heat builds up exponentially in an aired-down tire at highway speeds, and heat is the enemy of your tires. Take care of your tires and they will take care of you!

When in doubt, start with the placard-recommended psi and drop 10 psi for your first trip out. You will notice a huge difference in ride quality and traction performance. Just as important is airing back up when you get back to the highway.
Most of us add a lot of weight to our Jeeps—armor, bumpers, winch, camping equipment, recovery gear, and so on. Jeeps come from the factory with tires that most of us quickly upgrade, but you can still default to placard specifications for tire pressure until you determine the ideal pressure for your updated rig.
Before TPMS, some Jeep owners simply eyeballed the bulge in the sidewall to get their tire pressure where they wanted it.
A tire aired down to 10 psi is about as low as we go with heavy JKs without beadlocks. And that is only for crawling, not running dunes or fast dirt roads.
Let’s review the math. The weight at each of the front and rear axles is 2,200 pounds. The sidewall says that the max load is 2,405 pounds at 50 psi. That’s 2,405 pounds per tire or 4,810 pounds per axle. Jeep weighs 2,200 pounds at each axle, so 46 percent of max load per axle. But I add back in 20 percent for safety margin, so come to 55 percent of the max pressure of 50 psi. The number I come up with for a good street pressure would be about 28 psi. Then I take 35 percent of that number to arrive at my minimum off-road tire pressure, in this case about 10 psi. This would have been fine for rockcrawling, but clearly not fine for sliding laterally at 25 mph doing rainbows at Glamis.
A Hi-Lift jack with the Lift Mate, a sand base, a ratchet strap, and a good air compressor will make short work of an unseated tire bead. But make sure you clean the sand out of the bead and the valve stem as much as possible.

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