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300GE Cab, 6.9
556 Posts
More Guidelines than Rules . . .

cfowler - 2/8/2005 9:36 PM
What are the rules tire pressure?
When should you lower the tire pressure?
How low can you go?
Does it help to reduce against punctures?
The answer is different depending on conditions.

Ultimately, the point of airing down is to increase traction, as airing down increases the tire's contact patch.

Here's what I do, along with all the caveats:

On the street, I (almost) always run at the mfgr's recommended pressure.

On long stretches of washboard dirt road, I'll take it down to 15-18 p.s.i., which helps prevent the washboard from rattling my teeth out. On more technical stuff, I'll go down to about 12 p.s.i.. I've seen some folks go as low as 9. Maybe for rugged terrain the goal is that you air your tires down to the point that the whole width of the tread is on the ground - but none of the sidewall. With my tires, you have to let A LOT of air out to make that happen. I stop at 12 p.s.i. with a little bit of the edge of the tread still off the ground, because the tires feel just a little too mushy for my comfort if I go further.

When you air down the tires, while you increase the contact patch, you decrease the effective sidewall. Both my G's have 15 inch rims and 33 inch BFGs. That gives me a lot of sidewall to play with. But a G500 with 18" rims can't air down as much or else it will run the risk of damaging the rims or tearing the sidewalls. Speaking of sidewalls, if you're going to air way down, you need tires which are designed to take that abuse. BFG MT's and GoodYear MT/R's have multi-ply sidewalls for that reason. I'm not sure about punctures. I would reason that with tires aired down and the sidewall ballooned out, there would be an increase in the risk of cutting the sidewall on rocks, etc., I've seen cactus spines puncture sidewalls before. They are the weakest part of the tire.

When the tires are aired down you have to watch your speed, because the sloppy sidewalls will adversely affect handling at speed, especially on pavement.

It's also prudent to consider that aired-down tires will also reduce the axle-to-ground clearance.

If my truck is heavily loaded, I'll adjust the pressure upward accordingly.

It's also good to have a plan for airing back up when you get back to the pavement.

I'm sure there's more to say on the subject. With the guys I've driven with, tire pressure seems to be a very individual decision. YMMV.

I hope that helps.

— Spalding
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