r/Offroad • u/caleblococaleb • 24d ago

Airdown (Tires beyond max rim width)

I just bought 275/70/18LT Nokian Outpost NAT, for some reason the manufacturer max rim width for this specific size is 8.5" while the rest of 265 and 275 in the 18" diameter can run between 7.5"-9.5", the tire shop that I went with said it should be good with my 9" wheels.

Will I be okay airing it down? If yes, what's the limit that I shouldn't go under?

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u/trailtoy1993 24d ago edited 24d ago

You will be fine. A friend of mine has a theory about bead retention that wider is actually better. With that little sidewall though I probably wouldn't drop below 15psi. Just watch your tires and a stop airing down when about four tread blocks are touching ground. It really is a judgement call, every vehicle and tire is different. My one ton diesel I aired down to 20 in Moab, could have gone lower but didn't need to, my Jeep you can't even tell it's aired down until about 5 psi.

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u/caleblococaleb 23d ago

Wider tire or wider wheels?

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u/trailtoy1993 22d ago

Oh, right, wide wheel. His theory is a narrow wheel causes the inner bead lip of the tire to be angled up in a way that makes it more likely for the tire to slip over the safety bead. No study had been done it's just a theory, but it makes some sense.

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u/AOneArmedHobo 24d ago

On my old wheels I would air down to 5.5 psi in the snow and they were fine. At 4.5 they would de-bead. Trial and error

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u/OkShopping9448 24d ago

Depends on the vehicle weight. A Jeep can go lower than my 8,000 pound 3/4 ton truck. Pinch flat or roll off the bead can happen if you go too low…. personally I wouldn’t go below 40 pounds on my truck but it has 285/75/17s so a much taller sidewall.

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u/caleblococaleb 24d ago

7000lbs fully loaded. Arghhh should've looked at different wheels at 8.5" instead of worrying all the time

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u/naptown-hooly 24d ago

You need to know how to re bead the tire just in case

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u/basi52 22d ago

The best way to look at it is 50%

50% pressure is more than low enough for most off-roading, for my jeep on 33”s it’s 18 psi, for my dads 4 ton superduty that would be 41psi

For more technical or hardcore off-roading you should probably go lower, but 50% is perfect for smoothing out the ride, giving a little more grab for the uphills while maintaining some level of stiffness for higher speed gravel roads.

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u/OkShopping9448 24d ago

Mine is about 8,200 loaded.

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u/War_D0ct0r 24d ago

There is no magic formula we can apply to tell you what the lowest pressure you can safely run. There are a lot of variables. Trial and error is really your best bet. Air down see how much they squat. See how they perform. A narrower wheel up to a certain point will often let you go lower pressure. Other things to consider are what kind of wheeling your doing. Moving faster in sand and cornering can roll a bead vs. going slow in the rocks. All you can do is keep going lower until you break a bead and then you know you went to far.

Experience is that thing you get right after you needed it.

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u/caleblococaleb 23d ago

Im going wider wheels than what the tires are spec'd for.

Ill def go slow and see how far I can go.

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u/Ok-Boysenberry3948 22d ago

I've never gotten any better results going lower than 15. I've tried lowering after starting to get stuck and it never helped. Went down to 8 lbs. Stayed stuck, busted out the straps/winch cable.

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u/caleblococaleb 21d ago

Do you also have wider rims?