Not vehicle specific, talking about the minimum amount of pressure the tire needs to be able to withstand the heat buildup at highway speeds, even under light load.

It is my understanding that the JATMA, Japanese Automobile Tire Manufacturers Association and ETRMO European Tire and Rubber Manufacturers Assoc, both state a minimum pressure for Load Range E tires of about 46 PSI. Is anyone aware of this or have any info on possibly an industry standard, government mandate/regulation or anything like that?

 

I think it would have to be vehicle specific, or at least load per tire specific.

Don't know of any published material but I would say 30psi or above if they were on an empty Tacoma but if they were on a Crew Cab long bed 1 ton diesel truck I wouldn't run under 50psi.

I've run a load range E 10 ply Nitto Trail Grappler at 18psi all day long in the dunes on my V10 Excursion but that was mostly in the sand. I did make one trip on the pavement about 5 miles and set the cruise on 45mph just to be safe.


Update: I just found Michelin tires' load chart for a 235/85/16 Load range E tire (XPS Traction) and the lowest they show is 35psi.


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By "under light load" I assume you mean empty or close to it. In that case, the only factor is the weight of the vehicle. Therefore your question literally does not make any sense, since you are trying to remove the vehicle from the equation.

 

Im talking about a tire's iso-deflection state. The state where the deflection of the tire is just equal to its deflection at the maximum load, maximum pressure condition. The lower the pressure, the more deflection, and thus more heat buildup. There is a point at which the amount of heat produced is outside the parameters of what the tire was designed to handle. The lowest PSI that the TRA recommends for LT tires is 35 PSI. That 35 will support a certain load, but even if you don't need to support that much of a load, it is still recommended to run 35 PSI. But there is some question about the methods the TRA uses and, indeed, Japanese and European standards use different methods.

 

I'm a tire junky so I understand your curiosity but does it really matter, do you have a situation where you want to drive highway speeds with low pressures? I think you know the outcome.

Not trying to argue, I guess we're just trying to understand a situation that we think would never happen.

Here's a cool video I remembered while we're on the subject.

 

Yes! Tire junkies! Thats what Im looking for! Im on a fact finding mission here and its a very deep rabbit hole. Thats a warning.

So this all started with my other vehicle, a 2008 Toyota Land Cruiser. The vehicle comes stock with P285/60R18 tires. Toyota also offers accessories by its TRD line for the LC. One accessory is 17" wheels with BFG All Terrain LT285/70R17, load range E tires. (I bought a set of these tires/wheels off CL and started trying to determine what pressure to run in them.)

After a fair amount of research, I determined that originally, these tires were recommended to be run at 40 PSI by Toyota. They had a part number for a sticker that was to be installed when these tires/wheels are mounted stating the tire size and pressure of 40 psi. Then there was a recall/campaign because at these pressures, the TPMS wasnt tripping when it was supposed to. The instructions for the campaign were to recalibrate the TPMS and set the tire pressure to 46 psi. (A new sticker with the new pressure was made up with a new part number.)

Initially I posted the question to the LC forum, IH8MUD, and somebody responded that Toyotas recommendation of 46 PSI is wrong. He said it should be 39-40. He bases that on the stock tire and recommended pressure on the vehicles tire info tag. You take the P285/60R18, that toyota recommends be run at 33 PSI, from that you calculate the load needed to be supported at that corner, based on load inflation tables. But, when a P tire is fitted to an SUV or Light Truck, the load capacity of that tire has to be reduced by 10%, according to the Tire and Rim Association. So when Toyota fitted that P tire to the LC, at the pressure recommended, its actually 10% more than the load requirement for that corner of the vehicle. So when converting to an LT tire, you take the load that P tire and PSI combination gives you, subtract 10%, take that weight and apply it to the load chart for the LT tire to give you the right pressure. When adding in that factor, you get around 40 PSI. Without it, you get 46. Toyota originally recommended 40, now they recommend 46.

Toyota recommends 46 front and rear, even thought the GAWRs front and rear are a few hundred pounds different. BFG recommends 45 front and 48 rear, presumably due to the differences in those GAWRs, for that tire on that vehicle.

After I did some research, posted more info about the TPMS system, how it was designed, what the federal regulations require it to do, it was suggested that change from 40 to 46 was due to the fact that at 40, it wouldn't be in compliance with the federal safety standard for the TPMS.

In some other info I came across having to do with research conducted by the US government during design of TPMS systems, they found the Japanese and European tire industries say the minimum recommended pressure for LT E tires is about 58% of max pressure. (58% of 80 is 46.)

I also came across this fascinating study of tires being operated at less than maximum load. It says basically the relationship between load, pressure, deflection, etc.. is linear and that the correct pressure for a given load is just the percentage of max pressure that equals the ratio of load required to max load of tire, as in....

Pressure over Max Pressure of Tire = Load over Max Load of Tire

It also says that this is the formula that the TRA used to use to come up with its load charts, but moved away to another formula, and never explained why.

I just have a hard time believing Toyota and BFG are wrong about something like this. But when I contacted BFG and asked them about this, they didnt seem to know anything about that factor of 10%, which Ive seen explained elsewhere on the inter webs as well.

The 2008 and later 200 Series Toyota LC has a GVWR of 7300 pounds, and GAWRs of 3600 front and 4300 rear. I would think that it is rare that load range E tires are fitted to vehicles this light, because Ds are usually enough. Lots of Excursions running around with Ds. Im wondering if the recommendations of 46 PSI have to do with something else.

For example, I found a Tire Info Placard for a 2010 Chevy Suburban 2500 that came with LT tires LT 265/70R17 E. They recommend 50 PSI in the tire on the front axle with a GAWR of 4180. According to the TRA load inflation table, that tire with that pressure supports 2470 pounds. But half of 4180 is 2090, so it could run less pressure to support the weight, but they recommend higher.

To anyone still reading, is this making sense or just giving you a headache? I know Im throwing out a lot of stuff here, but my thought is that, when fitting LT tires to a vehicle that came with P tires, its more complicated than just a simple math problem.

 

From this link to FMVSS No.138.....


"The lowest inflation pressure used in the 2000 Tire & Rim Association Yearbook is 140 kPa (20 psi) for P-metric tires. In the 2001 Tire & Rim Association Yearbook, the 140-kPa pressures have been deleted, apparently because the Association believes they are too low for P-metric tires. The agency agrees that 140 kPa is too low and believes a floor is needed to assure that drivers are warned when tire pressure gets to or below that level. For the LT tires, we used the 2000 JATMA yearbook for the lower limits for Load Range C, D, and E tires. For most cases, the floor is about 58 percent of the maximum inflation pressure."

 

This quote from this NHTSA study....

"Low Pressure Endurance Test (Alternative 1)

This test is predicated upon the notion that a low pressure test would be most appropriate on tires that have completed the endurance test because a significantly underinflated condition for a tire is more likely to occur in a tire after several weeks of natural air pressure loss or due to a slow leak. The agency conducted 90 minute low pressure endurance test at 140 kPa (20 psi) inflation pressure, at a speed of 120 km/h (75 mph) and 100% load on the tires (2 samples of each of the 12 brands) that passed the endurance test. Similar tests were performed using the LT tires at 58 percent of their maximum sidewall inflation pressure. These low threshold values were selected based on the lowest inflation pressure at which a tire load is provided by the tire industry standardizing bodies. The results indicate that all 24 tires tested completed the test without failure."

 

OK I'm on board with everything you just said and I think the most simple answer here is, these guidelines, especially ones coming from vehicle manufacturers, are made by a group of people sitting in an office building along with their lawyers all of which who could not tell you the pressure in any of their own vehicles tires.

 

And I still consider myself a tire junky but you are pushing the limits of tire psycho.

 

Yeah I hear you, thanks for reading all that and responding. Im a little OCD when it comes to this kind of stuff. I have always kind of been into tires, but when I started looking into this I was amazed at everything I found about tire design and engineering. Its deep man. Anyway, I emailed a question in to Toyota on why they changed the recommendation from 40 to 46. They replied that they have received it and will respond asap, that was 6 days ago. We shall see....

 

Well Toyota responded with this....

Thank you for contacting Toyota Motor Sales, U.S.A., Inc.

We apologize for any confusion and appreciate you contacting Toyota Motor Sales, U.S.A, Inc. for clarification.

There are many factors considered when determining the recommendations and are based on the wheel and tire combination and how tires have been rated by the manufacturer as we do not manufacture tires.


Please contact the parts manager at your local Toyota dealer if you need further clarification regarding the tire and wheel combination for your vehicle.


Your email has been documented at our National Headquarters. f we can be of further assistance, please feel free to contact us.

Wow. Thanks. Im just trying to get an explanation for why they changed the recommendation.