To answer an earlier question: From a kind poster via PM in FJC forum:
07 TRD, came stock with 265/75LT16 BFG AT KO, Load Range D.
Door sticker, 35psi, all around.
Thanks for making this thread, Chris. I hadn't given the topic sufficient consideration before and it reduced my comfort level a little. It also led to a rather long search journey that exposed me to interesting information from straightforward sources like Tirerack all the way to obscure sources like the Federal Register, European Government documents, and technical books via book search. The fact that I'll be replacing tires soon and am strongly considering LTs again didn't hurt my motivation any. If relying solely on my own experience, I'd buy the BFG KOs Load C again in a heartbeat. It would be hard to exaggerate how much abuse they've survived but looking at risk a little more statistically compels me to re-evaluate my future needs and how I should care for any future set of LTs. Adding to my uncertainty is having no experience to draw from regarding the toughness of P-rated ATs. It's hard for me to believe I could have run Ps this whole time without any flats, but who knows. The tradeoffs between tire types is significant and deserves consideration. Even more so for people who live where it gets very hot.
I found many documents like the one you linked to and they all boil down to the same thing; namely, use load/inflation tables to make sure you don't go less than OE, which translates into adding 7-15psi to same size LTs. The 4Runner tire size puts it in the middle of that range. At issue is the extra heat generation/retention of LTs. I think this one-size-fits-all rule is a little unrealistic but I'd sure rather people follow a simple rule that doesn't always fit perfectly than replace Ps with LTs, run door sticker pressure, and take off on a fast, fully loaded road trip in the middle of a hot summer day. But if one wants to ease the ride stiffness of high pressure LTs without ignoring load/inflation tables, how should that be done? In seeking a reasonable way to use the tables, I decided I'd like to know how much load reserve was typical in an average OE installation. I made a couple of charts showing various 4X4 SUV (and one car) OE tire load/inflation relationships expressed as curb and GVW weight to tire load capacity ratio (all 4 tires). As the chart shows, a 4Runner V8 with 32" (265/70/17) LT replacements at 35psi still falls in the upper range of the group in load reserve. The chart also makes it easy to see how the FJ LT sticker pressure can be spec'd at 35psi even though that results in a reduction of load reserve compared to the OE P-rated tires.
I just realized that the FJC forum members tire is 265/75/16 as opposed to the 265/70/17 I used in the chart. That adds a touch of load capacity (20lbs per tire) but is essentially the same as 265/70/17.
The charts are pretty illustrative but they don't completely cover what a new LT tire owner should know. Speed is very important and speed ratings still apply but, in addition, the industry standards for LTs call for a 10psi increase from 65-75mph and then a 10% load reduction from 75-85. I wonder if there's not, perhaps, some difference between industrial use LTs and some of the popular consumer LTs, but even if there were, the design standards would be very similar so the message is clear: LTs aren't built for speed.
Something that none of the docs address. Tires are tested and rated at a brief snapshot in time, i.e., brand new. The P to LT warnings apply to brand new tires with full tread. Tread is the biggest heat generator on a tire, especially a deep treaded one, and the main contributor with tread is tread volume. As the tread on an LT wears down to P-like tread depths, the heat differences between P and LT should start to converge, though never completely as the LT carcass is still heavier and doesn't shed heat as well. The point here is it would seem extra attention should be paid to LT pressure when tires are new and maybe especially in regard to speed. Tires hold grudges and remember for the rest of their lives the times they were run too hot.
My new set of 32" LTs will be bought right after the summer heat is gone. Perfect timing. The fewer summers the better. I plan to run 36psi for everyday driving. If I take a long highway trip, I'll add a few lbs of pressure. If that trip takes place in the summer and I'm loaded up, I'll go more like 45psi (if load C) and watch my cruising speed. And I'll be a little more mindful of tread depth, speed, and temperature when driving aired down, especially on pavement (yeah, I do that sometimes). And considering the abuse the current set has taken to date, including 5 desert southwest summers, there won't be any more 80mph cruising until they're replaced. (For anyone wanting to use the tables, remember to divide the P load number by 1.1 (de-rating) to get a more accurate number to compare the LT load number to.)
There's a significant difference between the tire types that I think is under appreciated and is probably not helped by the retail tire industry. Tirerack, for instance, doesn't make the distinction clear in its spec charts (which leave a lot to be desired in accuracy, but that's another story). It's fairly easy to tell which is which if you know what to look for but why not just list P and LT in the tire sizes?
Things that would seem to be good ideas when going from P to LT:
1. Taking extra care with pressure when tires are new, especially in regard to speed, to avoid creating undue tread heat that can permanently affect the integrity of the tire.
2. Going up in tire size to help offset the load/inflation reduction.
4. Picking a tire from a major manufacturer.
3. Switching to LTs for appropriate usage reasons -- towing, off-roading, etc -- like people do with snow tires.
Things that may be less than good ideas:
1. Switching to an LT soley for looks.
2. Switching to LTs for someone who likes to drive very fast.
3. Taking a long, fast, loaded summer road trip with same size, switched out, new LTs at sticker pressure especially in a vehicle whose OE tire size offers less than average load reserve.
So, in a nutshell and imho, a new LT owner not willing to ride around on hockey pucks all the time should pressurize to a load reserve that resembles a typical OE installation and then tweak for speed/load and/or hot weather.
:guitar2:♫ Got rid of my high maintenance girl, but then I got LTeees ♫ :guitar2:
