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Old 07-10-2018, 03:10 PM   #41
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Originally Posted by FOURWHL View Post
Just to be clear here I’m not recommending anybody run exactly what’s in the tables. As mentioned by others the weight of your RV is a moving target and some margin of error (above what’s listed) is just logical. I choose to believe the owner of the coach who’s loading it is capable of making that decision as to what is a comfortable margin.

Now as to the “why wouldn’t you just run max pressure” question-

What some of you may be missing is these dynamax rigs specially on the freightliner chassis have between 5,000 and 7,000 lb CCC. That’s a ton of variability. I’m with you in a trailer, max pressure makes perfect sense. Even on an RV that is running near GVWR. But when COMPLETELY loaded with the tounge weight of my trailer I’ve still got 2,000 lb on the rear axle and 3,000 on the front running full pressure is just not warranted. Recommending everyone do such is just going to result in a bunch of unhappy campers (pun intended)

If it would please the courts I’m happy to post up all my weights stickers and such for continued judgement
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Old 07-10-2018, 06:10 PM   #42
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I've got some further questions here... FOURWHL, I think you can relate to my use case.

So some of the places I will be going (off road parks...) require travel down dirt roads to get to the camping area. These roads are bumpy to say the least. I plan to go very slow.... but there are a few rocks on them and I was intentionally thinking a slight air down might help when it is over 1 mile in.

Anyway, in the off-roading world we run very low tire pressure all the time. Radials that run at 40psi on the street are often lowered to 15psi to both improve traction and ride, as well as to avoid puncture damage. I run some of my bias plys as low as 4psi but that would never work for a typical radial.

Anyway, these same tires would be destroyed and dangerous on the road at speed with that low pressure, but low speeds are fine even though the tires temporarily deformed greatly during use. These are obviously a different type of tire, and this is why I'm asking for clarification on how different they are. My concern is damaging the tire on rough surface with too much pressure and no give in the tire, but it might be worse to lower the pressure if this particular style of tire construction has a major issue with any significant deformation. I have trouble grasping the idea that they are all that different... but maybe.
I would not recommend lower pressure in the situation you describe. I used to drive dump trucks carrying many, many more tons of weight than your rig has and we never lowered air pressure when off road. Theoretically you get a little more float on soft ground with pressure lowered, but once you get into these weight ranges the benefit is minimal. Plus you run the risks below.

Remember too that typically when you go low pressure on off road rigs you have bead lock wheels. You don't want to unbead a tire on your motor home or spin the wheel inside the tire. Plus, you run the risk of puncturing a sidewall if you lower the pressure too much. You want to be running over rocks with the tread and not the sidewall.

And, with lower pressure you will get more sidewall flex/squirm (increases the side-to-side sway), lower ground clearance and a greater risk of dragging something underneath especially if the coach is swaying. Suffer the slightly rougher ride and just go really slow is my advice.
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Old 07-10-2018, 07:12 PM   #43
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I'll offer up my .02 for whats its worth. I tend to lean towards using maximum rated pressure versus lower PSI's.

To FourWheels point, there is typically a safety margin on CCC with these big rigs, but that is only when you are going straight down perfectly level roads. If you are only 10% above recommended inflation based on axle weight and you drive on a severely crowned road (or rutted pavement), the majority of the weight on a given rear axle end can be unevenly distributed between the dual wheels. In this scenario the inner dual can be carrying most of the weight and be overloaded quickly and start getting hot. Similar situation is possible on a flatter road if you are in a significant and consistent crosswind that is leaning the coach over for an extended period. Note this is why pressure sharing devices like the Crossfire system work to minimize this impact.

The discussion also does not address the impacts that lower pressures have on handling. Lower pressures will result in more sidewall flex and sidewall squirm. Flex will magnify crosswind and cornering lean, this will effect handling as well due to geometry changes to the front axle steering. Note-If you look under a 1920's Model T and your DX3, you will see that the suspension design and setup is identical = leaf springs, shocks and straight axle. Same to some degree on the rear axle other than the airbags. This design is cheap to manufacture, durable and can handle a lot of weight. But it will cause the steering to hunt and wander when leaning due to crosswind, off camber road surfaces and turns. Sidewall flex magnifies the effect.

I agree with the statement regarding contact patch being the same on two identical constructed tires with the same PSI, but one is skinny and the other is wider. But again, this only applies when going straight and level. I build race car chassis and drive competitively. I've worked a fair bit with Hoosier's engineers on the R7 radials I run on my specific car, weight, chassis setup. I run an 8" wide tire up front with 34 psi cold, 10.5" wide rear with 30 psi cold. Higher pressures are better in radials used in road racing because of the severe side loads on the tires. These same size tires in a bias ply would be run at 20 psi front and 24 psi rear. The higher psi in the radials prevents the sidewalls from rolling under in hard cornering and ensures the tire retains the full contact patch versus lifting the inside edge. Thats also why wider tires will keep adhesion better than the skinny tire will in the same situation. The wider tires will also consistently maintain a larger contact patch when rolling over small stones or other imperfections in the paved surface.The radials, when properly inflated, also do not heat up as much as the bias ply. The fronts suffer the most abuse due to both lateral G's and off angle scuffing when turning. The rears have to handle the forward push of the car when accelerating and some lateral G's when cornering hard. The higher pressures also ensure straighter and more stable steering when braking hard when most of the weight transfers to the front end.

I understand no one is racing their motor home, but every impact discussed here occurs the same on your motor home, its just more magnified when racing at higher speeds. There are a lot of other factors too like tire compound stickiness, negative camber, etc. but those usually are not a factor in RV tires.

So I always recommend new RV owners to start off at maximum psi per the tire sidewall numbers. See how the rig rides and handles when fully loaded. If the ride is too rough, lower the PSI in increments of 5 psi all around and do the same test drive over again. As a rule of thumb I would never lower more than 10% below maximum unless you weigh each corner of the rig when loaded and adjust accordingly. On the DX3 maximum is 100 psi so I would never go under 90 psi. Even if you weigh each corner, take these other factors into consideration and give yourself some safety margin to account for them. If the handling begins to get squirrelly, dont go any lower regardless of the axle weights.

If you stay on the high side of PSI you may have a slightly rougher ride and theoretically a very small chance of higher tire wear in the center. If you go towards the lower PSI you run a greater risk of negatively impacting handling, reducing MPG and also increasing your overall tire wear not to mention the potential safety impacts of accidentally over loading a tire and over heating it or causing a blowout.
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Old 07-11-2018, 10:05 AM   #44
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FWIW, our DX3 weighed 26840lbs loaded with the 5 of us, full of fuel and DEF, and with about 30 gallons of fresh water. The weight was split 9940lbs on the front axle and 16900lbs on the rear.



The max pressure on the sidewall of the Michelins is actually 110 psi. Originally, I ran this in all the tires. The ride was harsh. Based on the weights above, the Michelin documentation recommends 85 psi in the front tires and 75 psi in the rear. I ended up lowering the pressure in all the tires to 88 - 90 psi. The ride is WAY better, and I believe the coach actually handles better.
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Old 07-11-2018, 10:12 AM   #45
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Yeah I run 90 front 100 rear because with my tongue weight I was closer to 18k rear
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Old 07-11-2018, 05:06 PM   #46
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Another important point about setting your tire pressure...



After weighing your rig and looking up the needed pressures for the specific tire manufacturer and model tire, measure the "cold" pressure early in the morning and out of the sun... Sun shining on the tires will cause a significant pressure increase.



Also, weigh your rig with the coach fully loaded for a trip - water, black and grey tanks full (or just add the weight of about 8.5lbs/gallon for empty tanks).



Once you have your axle weights, look up your axle weight ratings to determine if you are overloaded on your axles. Nothing like a bent or broken axle to ruin a trip...



Your tire pressures for front vs rear tires are most likely going to be different.



If you want your tires to last longer, don't park them for storage directly on concrete. I use sheets of plywood on the concrete floor to separate the rubber from the concrete. I believe this is to prevent the concrete from drawing out the moisture in one spot of the tire (over long term parking) and causing premature tire failure. (This is straight from the Michelin website)...



Happy Travels,

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I am not usually negative but this is BS....how ignorant can you be??? Weight considerations are not in ounces....duhhhh
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Old 07-11-2018, 06:49 PM   #47
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I am not usually negative but this is BS....how ignorant can you be??? Weight considerations are not in ounces....duhhhh
You are quoting another member and reference ounces I have read their post what are you referring to?
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Old 07-11-2018, 07:08 PM   #48
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You are quoting another member and reference ounces I have read their post what are you referring to?


You are correct, I apologize. I did quote the wrong member. Obviously you know the one I intended to quote.
I really get aggravated when I see BS like that with no basis. Just usually don’t say anything.
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Old 07-12-2018, 12:31 PM   #49
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I am not usually negative but this is BS....how ignorant can you be??? Weight considerations are not in ounces....duhhhh
Even where we disagree or even if it APPEARS someone is WAY off track, it's better to start off nice. A lot of good info is being exchanged, some of it quite technical, and mistakes and misunderstandings are likely to be made in both transmitting and receiving.
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Old 07-12-2018, 07:50 PM   #50
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Thank you!

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Originally Posted by Tireman9 View Post
1. I suggest you read my RV Tire Safety blog post of March 4, 2013, that covers how to adjust inflation when tires are not "cold"


2. You might bookmark my blog, or subscribe (it's free) as I am fairly confident I have probably already covered any of your tire related questions. There is a topic index on the left.
Tireman9 - I just clicked the link to your blog and read a few of your articles. Thanks so much! Impressive blog about tire safety. I have bookmarked it. Bev
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Old 07-12-2018, 09:30 PM   #51
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I know this topic has been discussed ad nauseum, and I have searched the forum but I can't find the succinct answer I'm looking for, yes, its about tire pressure (again). Is the consensus that you fill to the amount listed on the yellow sticker on the coach door jam, not to the pressure listed on the tire? We have a compressor that is supposed to fill to 100psi but seems it won't fill the tires past 80 and the sticker says 95 so we have to do it at a truck stop or something. However, aren't you supposed to do it when the tires are cold? So if you stop on the road, and the tires are hot, what to do? Thanks!
Easy way to do it. Near the end of the day on the road check the pressure and add, say, ? # to get it above what you want. Then in the morning bleed back with cold tire pressure to what you want. .
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Old 07-13-2018, 04:09 PM   #52
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Very interesting. Thanks for the tips.

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Old 07-15-2018, 01:59 AM   #53
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I think everyone agrees that underinflation = dangerous.

Overinflation can be just as dangerous. In fact, WGO, MB, Dodge, Michelin -- and I assume, most if not all other tire mfrs -- warn against both under and over-inflation.

Obviously there are degrees of over and under-inflation.

It's safe to assume that the tire mfrs that publish load & inflation tables have a safety margin built in to the recommended pressures. They are acutely aware that they would be sued into bankruptcy if the recommended pressure was too low, or right on the hairy edge -- resulting in numerous fatal accidents. Their legal depts are making sure they are playing CYA.

So the pressures in the tables are perfectly safe. The engineers take into consideration:

* Moderate ambient temp changes.

* Changes in altitude.

* Variations in pressure gauge accuracy.

* Roads are crowned and have curves.

* Loading from side winds.

* Modest increases in loading.

* Typical hwy speeds.

* Etc.

They must. It would be preposterous to publish a table that is based on: no changes in elevation; gauges with perfect accuracy; San Diego like temp changes; no winds; no curves; speeds <45 mph; and no crowned roads.

So the published pressures are perfectly adequate.

If a tire gauge is off a few %, or the tires are slightly warm, and they end up 3% low, they aren't going to explode.

Likewise, if the gauge is off the other way, or there is some other factor and the pressure is a few % over the recommended pressure, no problem.

It's when under and over-inflation become extreme 15-20%+ either way that it gets dangerous.

To be clear, if someone said I had to choose between 20% over and 20% under I'd choose 20% over-inflation every time. Luckily, we do not have to make that choice.

Generally speaking most people are at least vaguely aware that low tire pressure is dangerous.

What is not common knowledge are the reasons why over-inflation is also potentially bad/dangerous:

* Increased chance of blowouts from sharp impacts (potholes, etc).
* Increased chance of punctures.
* Smaller contact patch means longer emergency braking distances and potentially worse emergency handling.
* Harsher ride.
* Increased tramlining.
* Increased center tread wear.

Needless to say, a few % over the tire mfr's recommended pressure is no big deal -- but over the ~10 years I've been a member of RV forums, it has become obvious that many owners think that "If under-inflation = bad, severe over-inflation = good!"

Many owners and even tire shop employees are so clueless they think that the max. pressure on the sidewall is the proper pressure -- regardless of load.

At the end of the day, people are obviously going to do what they want. I don't want to argue and I'm not attempting to tell anyone what they should do. For DW & I though, it makes the most sense to simply follow the recommendations of Michelin (in our case). Like most people, we are not in a position to second guess the engineers from one of the world's largest, most respected tire companies.
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Old 07-15-2018, 02:12 AM   #54
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From Michelin

https://www.michelinb2b.com/wps/b2bc...s_Brochure.pdf

"Overinflation, on the other
hand, will reduce the tire’s contact area with the road
surface, which reduces traction, braking ability, and
handling. A tire that is overinflated for the weight it is
carrying is more prone to a harsh ride, uneven tire wear,
and impact damage."
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Old 07-15-2018, 03:20 AM   #55
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I think many get confused between an "ST" tire which is designed to be run at max sidewall pressure...

But then... Many replacement "ST" tires for 30 ft (plus or minus a few ft) RVs are only available in either LR D or E. These RVs were originally equipped with (205/225) LR C tires running at max sidewall pressure. A LR D or E replacement tire will not need to be operated at that tire's max pressure. That's why most reputable suppliers publish a load/inflation table.
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Old 07-15-2018, 06:24 AM   #56
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Originally Posted by sajohnson View Post
I think everyone agrees that underinflation = dangerous.

Overinflation can be just as dangerous. In fact, WGO, MB, Dodge, Michelin -- and I assume, most if not all other tire mfrs -- warn against both under and over-inflation.

Obviously there are degrees of over and under-inflation.

It's safe to assume that the tire mfrs that publish load & inflation tables have a safety margin built in to the recommended pressures. They are acutely aware that they would be sued into bankruptcy if the recommended pressure was too low, or right on the hairy edge -- resulting in numerous fatal accidents. Their legal depts are making sure they are playing CYA.

So the pressures in the tables are perfectly safe. The engineers take into consideration:

* Moderate ambient temp changes.

* Changes in altitude.

* Variations in pressure gauge accuracy.

* Roads are crowned and have curves.

* Loading from side winds.

* Modest increases in loading.

* Typical hwy speeds.

* Etc.

They must. It would be preposterous to publish a table that is based on: no changes in elevation; gauges with perfect accuracy; San Diego like temp changes; no winds; no curves; speeds <45 mph; and no crowned roads.

So the published pressures are perfectly adequate.

If a tire gauge is off a few %, or the tires are slightly warm, and they end up 3% low, they aren't going to explode.

Likewise, if the gauge is off the other way, or there is some other factor and the pressure is a few % over the recommended pressure, no problem.

It's when under and over-inflation become extreme 15-20%+ either way that it gets dangerous.

To be clear, if someone said I had to choose between 20% over and 20% under I'd choose 20% over-inflation every time. Luckily, we do not have to make that choice.

Generally speaking most people are at least vaguely aware that low tire pressure is dangerous.

What is not common knowledge are the reasons why over-inflation is also potentially bad/dangerous:

* Increased chance of blowouts from sharp impacts (potholes, etc).
* Increased chance of punctures.
* Smaller contact patch means longer emergency braking distances and potentially worse emergency handling.
* Harsher ride.
* Increased tramlining.
* Increased center tread wear.

Needless to say, a few % over the tire mfr's recommended pressure is no big deal -- but over the ~10 years I've been a member of RV forums, it has become obvious that many owners think that "If under-inflation = bad, severe over-inflation = good!"

Many owners and even tire shop employees are so clueless they think that the max. pressure on the sidewall is the proper pressure -- regardless of load.

At the end of the day, people are obviously going to do what they want. I don't want to argue and I'm not attempting to tell anyone what they should do. For DW & I though, it makes the most sense to simply follow the recommendations of Michelin (in our case). Like most people, we are not in a position to second guess the engineers from one of the world's largest, most respected tire companies.
Well said. A lot of my thoughts as well
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Old 07-15-2018, 01:16 PM   #57
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Originally Posted by sajohnson View Post
https://www.michelinb2b.com/wps/b2bc...s_Brochure.pdf

"Overinflation, on the other
hand, will reduce the tire’s contact area with the road
surface, which reduces traction, braking ability, and
handling. A tire that is overinflated for the weight it is
carrying is more prone to a harsh ride, uneven tire wear,
and impact damage."
True, but perhaps misleading, because that's their ENTIRE warning on overinflation. You'll see 10 or 20 times more warnings about UNDERINFLATION, including permanent casing damage and the catastrophic types of sudden failures. Underinflation risks are categorically different than overinflation risks.

Quote:
Originally Posted by sajohnson View Post
I think everyone agrees that underinflation = dangerous.

Overinflation can be just as dangerous.

***
To be clear, if someone said I had to choose between 20% over and 20% under I'd choose 20% over-inflation every time.
***
We may indeed all agree that underinflation is dangerous. Maybe we don’t all agree “overinflation” is “just as dangerous” or that “overinflation” presents the same category of risk. I’ll bet there is near unanimous agreement that it’s better to higher than lower. That is the takeaway that I've been advocating.
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Old 07-16-2018, 07:03 PM   #58
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Observations from a tire engineer.


I have seen literally thousands of tires that failed or were significantly damaged from running 9 to 15 psi low.
Conversely while overinflation can in theory result in an increase chance of an impact break most impact related failures were not related to "over-inflation".


Here is an example from my wife's carand HERE is one on my car.


Many times the above 2 examples would have been driven till the sidewall ruptured and the tire destroied with the driver claiming they never hit anything and the tire must have been "defective"


While significant over-inflation can be a problem the tires I have seen where high inflation was the direct cause were over-inflated by more than 125%.


Calling overinflation "dangerous" is a bit like crying "wolf" IMO.


Proper inflation for motorhomes is not the same as proper inflation for trailers as the forces involved in the belt separations are significantly different. You can real about 'Interply Shear" problems with multi axle trailers.


I advocate that tires in trailer application run the tire sidewall inflation. This also assumes the owner has confirmed they have at least a 15% margin of tire load capacity over actual measured tire loading.
Motorhomes should inflate tires based on the heavier end of eaxh axle (or use 53% of the axle weight if you don't have axle end weight)
Then consider the L&I table inflation as the MINIMUM cold inflation pressure and add 10% to that number for your "set" pressure.


None of the above is for off-road or racing or driving North of the Arctic Circle.


If you are only going to read one of my blog posts try THIS one.
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Old 07-16-2018, 08:13 PM   #59
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Originally Posted by Slow Moon View Post
True, but perhaps misleading, because that's their ENTIRE warning on overinflation. You'll see 10 or 20 times more warnings about UNDERINFLATION, including permanent casing damage and the catastrophic types of sudden failures. Underinflation risks are categorically different than overinflation risks.



We may indeed all agree that underinflation is dangerous. Maybe we don’t all agree “overinflation” is “just as dangerous” or that “overinflation” presents the same category of risk. I’ll bet there is near unanimous agreement that it’s better to higher than lower. That is the takeaway that I've been advocating.
I think we're on the same page for the most part. As I said:

"To be clear, if someone said I had to choose between 20% over and 20% under I'd choose 20% over-inflation every time."

Both have an element of risk, but a tire that is under-inflated by 20% is more likely to fail than a tire that is 20% over.

We seem to have consensus there.

The potential consequences of under-inflation are usually more severe than those from over-inflation.

Forced to ignore the mfr recommended pressure and pick between higher or lower, yes, higher is better -- but that is a false choice.

Tire mfrs spend countless man hours and millions of dollars creating their load & inflation tables. Unless we hear otherwise -- from the tire mfr; the tire industry; or the NHTSA -- I see absolutely no reason to deviate from the mfr recommendations.

As I said above:

"It's safe to assume that the tire mfrs that publish load & inflation tables have a safety margin built in to the recommended pressures. They are acutely aware that they would be sued into bankruptcy if the recommended pressure was too low, or right on the hairy edge -- resulting in numerous fatal accidents. Their legal depts are making sure they are playing CYA.

So the pressures in the tables are perfectly safe. The engineers take into consideration:

* Moderate ambient temp changes.

* Changes in altitude.

* Variations in pressure gauge accuracy.

* Roads are crowned and have curves.

* Loading from side winds.

* Modest increases in loading.

* Typical hwy speeds.

* Etc.

They must. It would be preposterous to publish a table that is based on: no changes in elevation; gauges with perfect accuracy; San Diego like temp changes; no winds; no curves; speeds <45 mph; and no crowned roads.

The published pressures are perfectly adequate."

A few psi over won't hurt, but beyond a few % it's likely to be worse overall than the recommended pressure.

Some people seem to think that the recommended pressures are only marginally safe -- that they are the absolute bare minimum, and using them is somehow 'flirting with disaster'. That Michelin and Goodyear engineers cannot be trusted. Consequently, they over-inflate their tires needlessly -- sometimes to the maximum pressure on the sidewall, which may be 50% or more over the recommended pressure.

One thing we know for sure and that is that corporations do not like to pay out hundreds of millions, if not billions, of dollars as a result of lawsuits. Therefore, they will absolutely build a safety margin into their recommended pressures, so that there is no way they can be successfully sued by those who use their inflation tables.

We run Michelin's recommended pressure in all of our tires. The wear is even and they have never gotten hot.

There is a rule of thumb about tire pressure -- perhaps you're aware of it. IDK how accurate it is but supposedly if a tire is properly inflated (cold) the pressure when hot should be 10% greater. FWIW, our tire pressures go up by exactly 10%.

BTW -- we use a Cat's Eye system on the duals (which might account some for the extremely even wear), and we have a TireTracker TPMS. I also check the tires at every stop.
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Old 07-20-2018, 12:22 PM   #60
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So here's another curveball in all of this ... How can you trust your tire gauge.

I recently installed a TST 507 and was surprised and a little concerned to see it reading 10 psi above my gauge. I went out and bought another gauge as a tie breaker. That one read 5 psi above the TST! So now I've got a 15 psi spread. Sheesh.

All this precision and were missing on accuracy
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