LT Tire Pressure

[Muddy Duck]

I know this topic has come up often, so I emailed Michelin. My question and their answer are as follows:

I drive a 2010 Toyota 4Runner which originally came with P265/70R17 tires and Toyota recommends 32psi for these tires. I now have LT265/70/R17 (E Rated) Michelin LTX M/S2's. Should they be inflated to a different pressure? The GVWR is 6300lbs. Also? Should I increase the pressure when towing a 4000lb trailer? Any help would be appreciated.

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When installing a different size than the original equipment tire, all vehicle manufacturer specifications must be maintained. The replacement tire should be inflated to provide the same load capability of the original tire size at the manufacturer’s recommended pressure. By changing to the LT rated tire your pressures should now be at 50PSI. You do not need to change the pressures for extra loads or towing.

 

[bimrunner]

Yes and yes. Play around with the inflation to see what it likes. My tires are bigger at LT285/75/17 and I run 45 normally and add for towing up to 50. You are probably going to want to be closer to 40 with your tire size but it is really one where you are going to just have to try out a few inflations and see what you like.

 

[fattire]

Curious...is there a calculator for this somewhere? I seem to remember a detailed 1engineer thread discussing it but the tire sizes were different from what I run (285/70/17).

I run them at 42 now. Seems okay but really don't know any better. I kind of changed everything at once (wheels, tires, suspension) and have very little baseline on the new setup.

 

[Omar RVA]

[MENTION=809]fattire[/MENTION] , you may be thinking of this thread: http://www.toyota-4runner.org/engines-suspension-wheels-tires-audio-accessories/110299-lt-tires-need-more-air-pressure.html. It has a solid and in depth explanation in the first post. It's also incredibly relevant to OP's question.

OA

 

[fattire]

That's the one. Thanks for reposting it! It's difficult to do advanced searches from an iPhone. Also, Apologies to JB for attributing it to 1engineer.

 

[Muddy Duck]

I originally studied the above mentioned thread and had settled on 44 psi. My mechanic who installed the tires insisted that the OE psi from the door sill sticker was best. With all of the differing opinions I thought I'd ask Michelin. I've driven around today with 50 psi and my MPG's have improved. The ride is a bit harsher so I'll keep fiddling with it.

 

[Ol'Baditude]

My mechanic who installed the tires insisted that the OE psi from the door sill sticker was best.

Unfortunately this seems to be quite common. I have to get the service rep to add a note to the work order every time I have my runner if for service at the dealership now so they don't mess with my air pressures anymore. They used to set them back to 32psi each time I was in for service - as though they were the same P metric Bridgestones that came stock - and each time I would drive away noticing how squishy the ride was. IMO It's just not safe or reasonable to have the carcass of an LT tire inflated to 32psi, and expect it to do its job in terms of load carrying capacity and vehicle handling. Incidentally, I run my LT tires at 38psi for daily driving and 42-44 psi loaded, depending on the actual load. For me, these numbers give a good compromise between ride comfort, handling and load carrying capacity. YMMV

 

[bimrunner]

Unfortunately this seems to be quite common. I have to get the service rep to add a note to the work order every time I have my runner if for service at the dealership now so they don't mess with my air pressures anymore. They used to set them back to 32psi each time I was in for service - as though they were the same P metric Bridgestones that came stock - and each time I would drive away noticing how squishy the ride was. IMO It's just not safe or reasonable to have the carcass of an LT tire inflated to 32psi, and expect it to do its job in terms of load carrying capacity and vehicle handling. Incidentally, I run my LT tires at 38psi for daily driving and 42-44 psi loaded, depending on the actual load. For me, these numbers give a good compromise between ride comfort, handling and load carrying capacity. YMMV

I forgot to add earlier that I settled at 45 due to the weight of my armor and LT tires. I'd say 50 is pretty high and you'll probably like it best nearer to 40. However, it is somewhat subjective as long as you aren't riding on the center of your tires only. Contact patch is important.

 

[BlackWorksInc]

Toyota has a TSB for their LT equipped Tundras and FJs to replace the placard on the door with a new one stating 46psi for the tires.

I fill all my customer's LT tires to 46psi unless they specify a specific pressure when having their service done and simply quote Toyota's LT tire spec.

All that being said, I run 46psi in my Nitto Terra Grappler LT285/70R17's and have good results on my 4th gen.

 

[kowski49]

I also like 38 psi for daily driving.

 

[Misterquad]

Notice - I am not looking to start an internet argument.

I just have a theory and I wanted to conduct a test. And the premise is that you don't need to run a much higher pressure when you change from P-Rated to an E-Rated. I actually believe you can run close to the door label pressure. And to me the higher pressure for a stronger tire is actually counter intuitive. Anyway, on with my test.

So, I just conducted a personal test for the past 12,000 miles. And I had the Service Manager at Discount Tire to measure my results. And all four tires were very evenly and barely worn. He measured 16/32nds across all the tires. He measured 2 places on all four tires. When new the tires are 18/32nds. They were so evenly worn, that the Manager noticed on his own and was even impressed.

As for comfort and ride quality, I judge that through the seat of my pants. I like the ride and feel of the tires at this pressure. I feel that below 32 the ride is too mushy and above 38 it is too harsh.

So here is what I did. Last March, I bought a set of Goodyear, Duratracs, 265/70R17, E Rated and had them mounted and balanced. I drove them 12,000 miles without rebalancing or rotating them. I maintained a pressure of 35 psi. I did take the truck to get it aligned at time of installation and again at about the midpoint of my test.

I don't expect to change anyone's opinion but I was pleased with my findings. And even more convinced to stay with a lower tire pressure.

Thanks for reading this. - Brett

 

[1engineer]

Notice - I am not looking to start an internet argument.

I just have a theory and I wanted to conduct a test. And the premise is that you don't need to run a much higher pressure when you change from P-Rated to an E-Rated. I actually believe you can run close to the door label pressure. And to me the higher pressure for a stronger tire is actually counter intuitive. Anyway, on with my test.

So, I just conducted a personal test for the past 12,000 miles. And I had the Service Manager at Discount Tire to measure my results. And all four tires were very evenly and barely worn. He measured 16/32nds across all the tires. He measured 2 places on all four tires. When new the tires are 18/32nds. They were so evenly worn, that the Manager noticed on his own and was even impressed.

As for comfort and ride quality, I judge that through the seat of my pants. I like the ride and feel of the tires at this pressure. I feel that below 32 the ride is too mushy and above 38 it is too harsh.

So here is what I did. Last March, I bought a set of Goodyear, Duratracs, 265/70R17, E Rated and had them mounted and balanced. I drove them 12,000 miles without rebalancing or rotating them. I maintained a pressure of 35 psi. I did take the truck to get it aligned at time of installation and again at about the midpoint of my test.

I don't expect to change anyone's opinion but I was pleased with my findings. And even more convinced to stay with a lower tire pressure.

Thanks for reading this. - Brett

Man...
So you assume tire pressure is solely determined for even tire wear???
No, there will be no internet argument because you are so wrong you don't even have a starting point to argue.

I especially like your comment "And to me the higher pressure for a stronger tire is actually counter intuitive." So tell me, how long have you been designing tires?

Please please read these tables http://www.toyota-4runner.org/engin...es-need-more-air-pressure.html#post1005768and understand that your "E" rated tire at 32 psi has much less load capacity than a "P" rated tire at that pressure. This is an accident waiting to happen man.

Please don't spread stuff like this here, someone might actually believe it.

 

[Misterquad]

My considerations are based on contact area. And I am getting full contact area, proved that. And looking at the charts, I generally don't see a E rated tire chart for vehicles as light as ours. So, I extend the line for the LT curve.

I am being completely serious here and looking at the charts noted above. The 4Runner GVWR is 6,300 lbs. And so when divide by 4 it is 1,575 lbs./wheel. Assume an additional 200lbs. for uneven distribution. And use 1,775 lbs. static load per tire. I go to the curve for LT and extend the curve to intersect at 1,775 lbs. and it equates to 32.5 psi.

 

[Jeremy556]

Modern radial tires will have a very even contact patch side to side until getting into extreme ranges of under or over inflation. The contact patch gets longer with lower pressure, not wider like bias ply tires.

The recommended inflation pressure has a decent margin of error built into it, so you probably won't experience any failures at 36psi on an LT, with close to stock weight, just decreased tread life. At max GVWR, on a hot day, at high speed, you might.

You should try checking your tire temperatures with an IR thermometer after running on the highway at 36psi versus 42psi.

 

[BlackWorksInc]

My considerations are based on contact area. And I am getting full contact area, proved that. And looking at the charts, I generally don't see a E rated tire chart for vehicles as light as ours. So, I extend the line for the LT curve.

I am being completely serious here and looking at the charts noted above. The 4Runner GVWR is 6,300 lbs. And so when divide by 4 it is 1,575 lbs./wheel. Assume an additional 200lbs. for uneven distribution. And use 1,775 lbs. static load per tire. I go to the curve for LT and extend the curve to intersect at 1,775 lbs. and it equates to 32.5 psi.

Part of what [MENTION=66316]1engineer[/MENTION] is getting at is that while the contact patch may be fine, the load on the sidewall is not. I don't think the curve continues evenly at lower pressures, I would suspect it starts to flatten out by the lower pressures. Part of the issue is that underinflated LT tires tend to work the side walls too much from what I understand.

 

[scanny]

There were a lot of discussions about this topic, personally I went with user manual, which says that we need to take maximum load per axle from door sticker, divide by 2 and make sure that tire has pressure which is capable to support this weight. For stock size C load Duratracs it's about 35 psi, so I keep pressure about 36-37 just to make sure that it doesn't go less than 35 with temperature change. It worked fine for the past 3 years. Since 35 psi is capable to support fully loaded truck 32 psi might work for non-loaded T4R especially for less than highway speed driving, but it's risky in my opinion. And those numbers can be different for E tires or different size tires, correct values can be found in tire load table. Excessive tire pressure might result in premature wear and less contact with the ground. Just my 2 cents : )

 

[Misterquad]

I am looking at the curve noted on this thread and C,D and E follow the same curve and is pretty consistent over the full curve.

I did my calc above using GVWR but, my actual load per tire is more likely (4,700 lbs. truck, 185 lbs. me and 168 lbs. gas) or 1,263 lbs. per tire plus 200 lbs gives you 1,463 lbs. per wheel. Not even on the charts.

These numbers are consistent with my modded FJ Cruiser that I weighed on the truck scales. As a whole and tire by tire.

I even consider the sidewall discussion here. And it is much harder to get a side wall bulge on an E-rated tire than a P or C. I have witnessed aired down E-Rated tires that hardly show a bulged because the side walls are so stiff.

As for premature wear, my observation was 12,000 miles per 2/32nds of wear. Assuming this rate, these tires could go for over 80,000 miles.

I am trying to understand this. I hear this discussion everywhere but I interpret the curves differently.

 

[Misterquad]

I especially like your comment "And to me the higher pressure for a stronger tire is actually counter intuitive." So tell me, how long have you been designing tires?

Let me explain how I came to this conclusion.

As you recall from your Strength of Materials and Structures Classes in college, when you thicken a vessel it gets stronger and reduces the stress in the material.

So why would a thinner vessel be stronger than a thicker vessel?

What am I missing here? I am not seeing it.

 

[BlackWorksInc]

I am looking at the curve noted on this thread and C,D and E follow the same curve and is pretty consistent over the full curve.

I did my calc above using GVWR but, my actual load per tire is more likely (4,700 lbs. truck, 185 lbs. me and 168 lbs. gas) or 1,263 lbs. per tire plus 200 lbs gives you 1,463 lbs. per wheel. Not even on the charts.

These numbers are consistent with my modded FJ Cruiser that I weighed on the truck scales. As a whole and tire by tire.

I even consider the sidewall discussion here. And it is much harder to get a side wall bulge on an E-rated tire than a P or C. I have witnessed aired down E-Rated tires that hardly show a bulged because the side walls are so stiff.

As for premature wear, my observation was 12,000 miles per 2/32nds of wear. Assuming this rate, these tires could go for over 80,000 miles.

I am trying to understand this. I hear this discussion everywhere but I interpret the curves differently.

"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. 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." ​

Quoted from:
http://www.toyota-4runner.org/engin...s/110299-lt-tires-need-more-air-pressure.html [MENTION=2798]JB.[/MENTION]

I'm not trying to pick on you or anything, but rather point out with the highlighted areas where your logic is somewhat flawed. While many LT tires have very stiff sidewalls, that does not mean that sidewall flex/bulging is an indication of proper loading. For example look at Run-Flat tires that also have extremely stiff sidewalls, though these tires are designed to support the vehicle in the event of a complete loss of pressure, they can only do so for a limited amount of time and at a limited amount of speed because the plys in the sidewall actually start to break down from flexing and loading that is difficult to see unless you were strapped to the side of the vehicle.

I would suspect that the curve flattens out even more as pressures drop closer to zero (I'd actually like to see a chart of this myself, if some one has one), the reason being that after a certain point the tire can no longer effectively support the load and begins to degrade faster. Think of people who run under inflated tires that on the freeway just tear themselves to shreds, this is mainly due to the sidewall rubbing/flexing/shearing too much because of improper inflation/too much load; the result of many of these tires is blow-outs in the side walls that shred it.

Let me explain how I came to this conclusion.

As you recall from your Strength of Materials and Structures Classes in college, when you thicken a vessel it gets stronger and reduces the stress in the material.

So why would a thinner vessel be stronger than a thicker vessel?

What am I missing here? I am not seeing it.

Part of my statement above goes into this comment, but I would like to add an additional addendum to it (because your post actually slipped in while I was writing this ) This falls under the loading comments I made above so I won't rehash the details of low inflation and flexing too much, but rather impart that the sidewalls (as well as the tread) of a Radial Tire are not solid rubber; there is impregnated plys of steel, nylon, kevlar, ect. cabling that can separate from the rubber if given enough friction/shearing forces/flex/ect. Think of radiator hoses, over time they crack and deteriorate to the point that sections of the hose can be pulled off the inner nylon sheathing.

 

[Misterquad]

And I agree with everything you are saying. But these trucks have a per wheel loading between 1,463 and 1,775 lbs. I am just plotting those numbers on this chart.