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Old 10-02-2022, 06:08 AM
stevelikesrhino stevelikesrhino is offline
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stevelikesrhino stevelikesrhino is offline
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Join Date: Jul 2022
Location: New York, NY
Posts: 47
stevelikesrhino is on a distinguished road
Question Tire Pressure of LT Tires: Who should we listen to?

In case anybody asks, I have read every single post of the original "LT Tires Needs Higher Pressure" thread. Upon some digging, calculation, apparently across different models and tire sizes, the ratio between GVWR and OEM tire cold inflation load varies. For example, for the most common 265/65R17 that comes with 4th gen SR5, the OEM cold inflation load is at whopping 2055 lbs, 34% of the gross weight (fixed at 6005lbs, max gvwr for V8 4wd); however, for limited 18 inch rim, 265/60/R18 at the same 32 psi "only" supports 1985 lbs, only 33%. If that difference is tiny enough to not worry about it, check out the current 5th gen TRD Sport and Limited GVWR to OEM tire load: 1794 lbs, only 28.5% of the GVWR. Meanwhile back in 2014, the last gen FJ Cruiser comes with stock BFG KO, demanding 46 psi: that puts the cold inflation load up to 2318 lbs, 41%(!!!) of the GVWR. That means 4 tires combined, these tires support 164% of the GVWR of an FJ, must be designed to jump slopes with full camping gear and a tent and some rock sliders and a fridge.

It's easy to calculate a "safe" pressure from OEM tire pressure and load. As much as setting my E rated 265/65r17 KO2 to 44 psi is not quite unlivable, I do like to find a threshold for the tire pressure similar to the OEM door pressure, means I should not cross that line no matter what in normal tarmac driving. I have read about how underinflated LT tires can generate quite amount of heat, which may be devastating when you run them in harshest environment possible (death valley, Arizona, Gobi desert, etc.) when the sidewalls head up. The detailed explanation can be found at Nitto's Technical Bulletin, just Google Nitto TSB Tire Pressure. Running LT at 28 psi on public highway even for a 4runner is straight up dangerous, you're lucky if you have not run into any problem.

The final question is, who should I listen from a purely "safety" perspective? Is 28.5% GVWR enough of a headroom to not blow in Arizona heat? Because that puts my KO2 only at 36 psi, makes my already rough tires much more enjoyable, as long as I don't jump, hopefully.

Behind that, I understand Toyota engineers also adjust pressure for ride quality, handling, stability, tire wear and some other reasons, but when we put on the already rough but tough LT tires in replace of the original ones, do we accommodate that like these engineers does, and only put in enough load headroom to make these vehicles safe (strictly 25%+ GVWR)? Is that the borderline to not cross not matter what?


Steve


Pressure calculation source: Tire Pressure Calculator
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