LT tires need more air pressure.
 

...to match the load capacity of a P-metric tire.*

*A Standard Load tire (SL), usually P-metric (or metric, euro-metric -- a nearly identical twin). This is the tire found on nearly every new car, SUV, and Light Truck. On the sidewall it looks like P265/70/17 (or 265/70/17) as opposed to LT265/70/17.

How much more pressure?

• 10-15lbs, if an LT is switched for a P-metric tire with the same dimensions.
• LT load range (C,D,E) doesn't affect this.
• Tire size does.

This continues to come up and is often a source of confusion.

Not surprising though, considering that both of the following statements are true:

LTs carry less load than P-metric.
LTs carry more load than P-metric.

Throw Load Range into the discussion and things get nice and tangled. Describing the relationships in words doesn't paint a clear picture so I thought I'd try to make some -- pictures, that is.

The charts are based on tire size 265/70/17 and, in the 2nd chart, how it relates to the 5th gen 4Runner because that proved easiest, but the application to other generations is obvious and not too dissimilar. The curves should be reasonably accurate and more than good enough to illustrate the point. P-metric tires get load capacity de-rated 9% when used on an SUV and the curves reflect that. The tables used for reference come from this.




Hopefully it's easy to see how an LT can carry both less and more load than P-metric. It's all about air pressure and, for this tire size, it takes about 12psi added to the LT to match the load capacity of the P-metric tire. Hopefully it's also clear how pretty much the only distinction that matters is between P and LT. Load Range doesn't meaningfully affect the choice of air pressure for LT -- unless very high pressures are desired -- since the the longer load E curve is overlapped by the shorter D and C curves.There may, however, be implications for speed capability in the selection of load range. See Baldwin here.
http://i33.photobucket.com/albums/d8...1.jpg~original




Extending the line for LT down past where it ends (because load tables stop there) in this chart might seem to imply that making a pressure adjustment when switching to LT isn't important since it appears there would still be load reserve at 32psi even if no additional pressure was added, but that doesn't mean there's no benefit to adding more pressure (leaving it at 32 would be the equivalent of around 20psi in the stock tire). For one thing, the curve bends more as it goes down the chart towards zero psi, so calculating it as a straight line overestimates load capacity. Secondly, having plenty of load reserve is desirable since tires change over time -- and they don't get stronger. Lower pressures result in more heat which speeds the process. The cumulative effects of dynamic loading, speed, heat, age, hard use -- not to mention the inevitably lower speed rating in an LT -- are all good reasons the load reserve established by the factory tire at recommended pressure should be given due consideration. If nothing else, the level where the tpms triggers should be a good clue to that. Most of this is covered in the doc already linked. There is also the option to Ask a tire engineer - Toyota 4Runner Forum - Largest 4Runner Forum. The horizontal lines on the P-metric tires represent the ability to add pressure for special conditions (like high speed) but the extra psi assigns no additional load capacity. The language in load tables indicates that extra pressure for LTs at high speed is recommended, as well, and at lower speed thresholds than P-metric. The capability of a tire in both load and speed necessarily overlap each other.
http://i33.photobucket.com/albums/d8...2.jpg~original



Tire size makes a difference. More air equals more load capacity, whether it's a bigger tire or more pressure in a given tire. The grey curve extends the full distance, so it would be Load E
http://i33.photobucket.com/albums/d8...3.jpg~original

Reserve

Where did the charts originate? Copy this over to the 5G side please. I'm sure it will help some of those guys. Good stuff.

I'm under the impression that would be against forum rules.

Good catch. I'll edit the intro to include the source of the numbers.

Good work JB.

Whohooo 1E - charts, graphs, statistics, fax & figgers. :thumb3:

good charts. The only thing is not clear to me is a maximum pressure when tires still keep enough traction to make a stable ride and especially stable turns let say on rain. Becasue as I see it if I pump tires up at let say 50 psi I might like bumpy ride and it's going to be safe from load capacity point of view but I'm sure that I'll reduce traction at least by half. And I'm going to regret it if I have to make sharp turn on wet pavement. Or hit an icy path on the road.
But I guess that nobody knows it for sure.

Someone's on a tire research mission! This is good info, a gap in my auto knowledge apparently! Nice to fill it in some, I can make sure I'm at 22 PSI or greater when towing my boat now after some quick mental math... :P

Should be 5000lb 4 Runner / 4 tires = 1250 / tire, adding 200 per rear tire for a 400 lb tongue weight (obviously conservative by ignoring fronts) = about 1450 lbs / tire --> ~22 PSI?

Anyway, cool to see some numbers, take away seems to be there's plenty of room for error before things get bad... unless you have a Firestone Exploder and love to run 12 PSI ahahaha...

Was playing around with a small compressor in the garage a few weeks ago. One thing led to another and I ended up measuring tire deflection on the passenger front in 5lb increments from 80psi down to 5psi. The result is the graph below. One thing I wanted to know is what the curve looked like below 35psi where the load tables end for LT tires. I originally drew the lines in the charts, above, straight down to 20psi to make the relationships more obvious at a glance. They are a little too straight as it is, but continuing them straight past the end of the load tables needed to be fixed, and now it is.

It's interesting to me how 35psi is just about the spot where the curve below really starts to bend. I'm assuming it's no coincidence that's where the LT load tables end.

http://i33.photobucket.com/albums/d8...e.jpg~original

idk where you got those graphs but they are way off... E rate tires typically hold 3600+lbs, at 80-85psi your graphs are showing tires maxing out at around 3000-3100lbs @80psi... thus throwing off your entire graph. having worked a both independent tuner shops and dealers, and been apart of several race teams and having also worked for a tire shop, i have been around tires of all makes types and styles, E rated tires can support the equivilant load of a P tire with lesser amount of psi. its in the construction of the LT type tire, now the same is true for D and C rated tires, although the gap is much less the lower the rating you go. also not all tires are created equal, one tire manufacture might say their D rated tire is able to hold 3450lbs while another manufacture will say 3500lbs... same is also true with the P tires as well, -and its not can also change within the same tire manufacture one style or design of tire might hold more or less than another(all in the same aspect ratio size)

just some food for thought. i dont know where you got the info, but its more than a little misleading...

Color me confused too. I check my street air pressure by looking to see if the tire bulges where it touches the ground, and if it does, I add air. My E rated KM2s require about 10 PSI less than the P-rated BigO Bigfoot ATs they replaced, so I don't get it.

Tsk tsk, I don't think you can visually check tire pressure :p

http://www.tirerack.com/tires/tirete...e.jsp?techid=1

I am not positive, but I think the max load for E rated tires really depends on the size and not the load rating...the chart @JB. seems pretty spot on for the 265/70R17 tire size...

I believe the numbers are correct for the 265/70/17 size. Other sizes in load range E will be rated higher, some less. I know for a fact my 285/70/17 BFG ATs (load range E) that were on my silverado were rated at 3195lbs @80 psi. The same size BFG ATs that were on previously (load range D) were rated at 3195lbs @ 65psi. I have seen some load range E tires with over 4000lbs rated capacity while others less than 3000. His chart seems ok to me.

Your post highlights why I made the thread. This continues to be an area of great confusion.

A careful reading of the first post and the linked load/inflation table should clarify things. The problem is, it kind of flips the things you posted upside down so it may be difficult to believe.

LTs carry less load (size and psi being equal) than Ps because they create more heat and the rubber, due to thickness and composition, is more prone to long term cracking when flexed.

There is also a link above to a Q&A page for a tire engineer.