I've seen lots of questions about how to determine the correct tire pressure for RV tires - but I haven't yet found an explanation of WHY you should inflate the tires to less than max pressure if you have more tire capacity than weight on them.

In our case, we had a wheel-by-wheel weighing done on our 5th wheel (by Escapees SmartWeigh). Our Load Range G Goodyears have a max pressure of 110 psi, but the tire load/inflation table for our tires says we can (should?) run them at 90 psi based on our measured axle weight.

So before I go letting air out of my tires, what benefit does reducing the pressure give me? I'm guessing it's better tread wear, because the tires won't be over-inflated for the weight that's on them? Or better handling? Something else? Are there any added risks that might offset the benefits? I'm just a little leery after having learned all the dangers of under-inflation.

By the way, I do have a TST 507 TPMS, so I can monitor tire pressures and temperatures while we're driving.

When Howard comes up for air from the last adventure perhaps he will provide more info as he is an expert on tires. But in general:

Assuming you are using the tire manufactures charts, air pressure for the highest load on the heaviest tire, plus 5 psi in some circles, is recommended for the best ride and tire tread life.  Simply put, the tire rolls on the pavement the way it was designed – not more in the middle of the tire so to speak.

If you look at the air pressure recommendation on the door jamb of your vehicle (car / truck) you will probably notice an air pressure recommendation less than the maximum tire sidewall psi rating.  That’s because the OEM’s maximum certified weight of the vehicle axle is less than the maximum rating of the tires and therefore a lower psi is recommended for the same reasons.

To comment a bit further:  Naturally this is a static, cold – meaning not having traveled, air pressure.  And to emphasize, the psi used should be based on the tire with the highest weight and that psi should be used for all tires.  It should not be based on a CAT or similar scale weight of an axle divided by 2.  It is very easy to have more weight on one side of the axle and therefor tire than the other.

All that said, within the maximum psi rating of the tire, better high than low.

HighwayRanger wrote:

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SO, until we can get the rig weighed wheel by wheel, I guess I'll back off a bit on the cold pressure of my G114s. When I set them at 120 psi, after driving for awhile, the TPMS tells me that they are up around 140 psi. So if I set them at about 110 cold, they should stay closer to their max pressure rating, right? By the way, we are at around 20K with two axles and the Mor/Ryde 8K IS suspension.

Roy

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Roy:

Actually, 110psi would be more than enough, cold, assuming your not overweight on an axle side. (As David said, don’t allow for heating.)  Your G114’s are rated at 4,805lbs per tire at max inflation of 125psi.  You’re not going to get close to 4,805lbs per tire with your rig unless you are seriously overloaded.  (But yes, I actually have seen 10K on one axle on a DRV Mobile Suites on the scales. Wasn’t pretty BTW.)

Per the chart at 110 psi that is good for 4.350lbs per tire. But as you said, I’d sure try to get a wheel-by-wheel if at all possible so you know the correct setting.  That said, assuming you had he maximum load on the 8K axle, that's 4,000lbs a side, that would be 100psi, plus the 5psi “extra” most recommend, or 105psi.  

Excellent choice of tire IMO.  Significant safety upgrade from the G614 "G" tires.

Bill

This quote is from the pages of Goodyear.

It’s important to have the proper air pressure in your tires, as underinflation can lead to tire failure.
"The “right amount” of air for your tires is specified by the vehicle manufacturer and is shown on the vehicle certification label and in the vehicle owner‘s manual."

What trailer owner’s must remember is no matter what tire is selected for use on their trailer (passenger, light truck, medium duty to heavy truck or special trailer tires) they are still going to be governed by the FMVSS. Recommended inflation pressures for the load carried are not a part of the FMVSS (Federal Motor Vehicle Safety Standards). In FMVSS a tire under inflated is below the vehicle manufacturer’s recommendations. A tire over inflated is one that has been inflated above the value found on the tire sidewall.

I’ve done this hundreds of different ways and all are - to some extent - misunderstood.

Every single one of the major tire manufacturers are going to give recommendations almost verbatim to the one from Goodyear above.

Industry standards generally form the basis for demonstrating product safety and quality before courts, regulators, retailers, consumers and others. That's not my statement. It belongs to NHTSA and is about tire industry standards.

Very basic info from tire industry standards; When plus sizing tires up from OE sizes found on the tire placard the first rule is to insure the replacements, provide at a minimum, the amount of load capacity the OE tires did at recommended inflation pressures. So the recommended inflation pressures for replacements will be the amount of inflation required to equal or exceed the load capacity provided by the OE tires.

Bill and Linda wrote:

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  That said, assuming you had he maximum load on the 8K axle, that's 4,000lbs a side, that would be 100psi, plus the 5psi “extra” most recommend, or 105psi.  

 

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 Why would you add an extra 5 psi?

TheNewhalls wrote:

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Bill and Linda wrote:

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  That said, assuming you had he maximum load on the 8K axle, that's 4,000lbs a side, that would be 100psi, plus the 5psi “extra” most recommend, or 105psi.  

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 Why would you add an extra 5 psi?

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 Other tire may experts have other opinions, but the tire experts I have worked with add the 5 psi to make sure the tire is inflated above the load to be carried.  Better high, assuming not above the max pressure allowed on the sidewall cold, than low.  That was likewise the advice I received from RVSEF.

A comment about running “lower pressure” than the sidewall maximum rating.  Many people, for safety, have replaced the marginally rated tires that come on many RV’s.  Many choose to upgrade their tires, especially the “G” tires to “H” tires.  When that is done there is no RV manufacture’s pressure recommendation to go by as the RV came with, for example, “G” tires – now one has “H” tires.  “H” tires, as do all tires, come with a recommended inflation chart from the tire manufactures which is also used, I might add, by vehicle and RV manufactures.  So many who know the weight on the tires choose to use the recommendation from the people who engineered the tires and add 5 psi to assure adequate inflation just in case the trailer gets a little heavy.  That was one of the reasons.

I will also comment that the marginal tires that come on many RV’s are so close to, or over, their maximum weight ratings when the trailer is loaded that the maximum sidewall pressure has to be the one RV manufactures recommend based on those charts.  That, BTW, is not always the case when a trailer is delivered with “H” tires. It depends on the rig and the number of axles, etc.  “It does depend.”

Following the tire manufacture’s pressure charts and adding 5 psi as was recommended, I have never had a blowout nor, according to the infrared heat gun I use every time we stop, do my tires ever get “hot.”  My last set of tires were still well above the manufacture’s minimum tread depth at 44,000 miles when I changed them and that was after trips to Alaska and Inuvik, NWT, Canada.  The latter was not a tire friendly road to say the least.  All I can say is, we must be doing something right taking the advice we have received.

Even though an owner chooses to use truck tires as replacements for their RVs does not change the fact that RV fitments are governed by the FMVSS. Type of tires used are not going to change that fact. Truckers tires are not so governed. The experts you talk with need to be experts with all safety regulations so they can apply those needed for RV vehicle fitments. All RV fitments, MH and trailers, above 10,000# are governed by 571.120. That includes recommended tire inflation pressures for OE tires and their replacements. There is nothing in there that says you cannot go above the vehicle manufacturers recommendations, but, industry standards will not allow going below the vehicle manufacturers recommendations. That's the gray area you enter when you determine your own safety below industry standards.

If you want an example of industry standards for truck tires I suggest you download and read the Michelin truck tire data book (PDF). It has step by step procedures for how truck facilities fit and inflate tires loaded for the road.

FastEagle wrote:

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There is nothing in there that says you cannot go above the vehicle manufacturers recommendations, but, industry standards will not allow going below the vehicle manufacturers recommendations. That's the gray area you enter when you determine your own safety below industry standards. 

-- Edited by FastEagle on Sunday 3rd of January 2016 02:06:54 PM

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 I could not agree more.

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  That said, assuming you had he maximum load on the 8K axle, that's 4,000lbs a side, that would be 100psi, plus the 5psi “extra” most recommend, or 105psi.  

 

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To determine a tire is under-inflated or over-inflated there must be a starting point. NHTSA has given total authority to set the correct (recommended tire inflation pressures) tire inflation pressures to all vehicle manufacturers that abide to FMVSS regulations. Once the Original Equipment (OE) tire fitments are determined and the recommended inflation pressures for them is set that information is put on the vehicle's federal certification label and affixed to the vehicle. It becomes the standard for any/all replacements. Basic industry standards for replacements is for them to have a load capacity equal to the load carrying capacity of the OE tires (by inflation). When using replacements with a different load/inflation chart than the OE tires it is necessary to determine a new recommended inflation pressure for them and make notations in the vehicle owner manual and adjacent to the certification label. It's in the standards, I've read them. Hardly any tire installer knows to do it.

Not looking to be antagonistic here, but I've never read anything about adding extra PSI above the recommended inflation from the inflation tables. I've had my motor home four corner weighed three times. All by RIVA standards. The first person recommended pressures from the weight tables plus 5 or 10 psi extra. My motor home handled poorly and was pushed all over the road by the semi's. The next two places I weighed my RV recommended only the pressures from the inflation tables. When I asked about adding an extra 5-10 psi they asked why would I add more than what was recommended? So, I didn't add anything extra and now my handling has improved dramatically and I don't even know when a semi is beside me until they are passing me. Maybe the "extra psi" is already incorporated in the tables. The mfgr max pressures are 100 psi, but I run 90 psi on the rear and 80 psi on the front.

TheNewhalls wrote:

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Not looking to be antagonistic here, but I've never read anything about adding extra PSI above the recommended inflation from the inflation tables. I've had my motor home four corner weighed three times. All by RIVA standards. The first person recommended pressures from the weight tables plus 5 or 10 psi extra. My motor home handled poorly and was pushed all over the road by the semi's. The next two places I weighed my RV recommended only the pressures from the inflation tables. When I asked about adding an extra 5-10 psi they asked why would I add more than what was recommended? So, I didn't add anything extra and now my handling has improved dramatically and I don't even know when a semi is beside me until they are passing me. Maybe the "extra psi" is already incorporated in the tables. The mfgr max pressures are 100 psi, but I run 90 psi on the rear and 80 psi on the front.

 

 

 

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 What does your tire placard say to run for the original tires?

Load inflation charts are written by tire manufacturers and approved by the TRA in the USA. They are in all respects for use by vehicle manufacturers and tire retailers. Vehicle manufacturers use them to select Original Equipment tires and set the recommended inflation pressures and tire retailers use them when replacement tires are of another size or differ in design from the OE tires. If an owner needs to use them the probable cause is over weight or load imbalance. Correcting the weight & balance problem will almost always fix the condition. 

On edit: A point to remember about my posts on this subject is I'm writing them from DOT tire regulations and tire industry standards. I'm well aware that many MH RVs are operating overweight just as many RV trailer are. The regulations and standards have zero provisions for those conditions. Not even some wiggle room. The following is a verbatim quote from NHTSA.

"The FMVSS have requirements for the manufacturer to use proper tires and rims for the gross axle weight rating (GAWR) and the gross vehicle weight rating (GVWR). The manufacturer may determine the GVWR by adding cargo capacity (if any) to the curb weight of the vehicle as manufactured. The wise consumer, before purchase, will determine if the vehicle has sufficient cargo capacity to carry the weight of water, additional equipment (such as televisions, and microwave ovens), and luggage. The manufacturer’s certification label must show the GVWR. The GVWR must not be exceeded by overloading the vehicle. There is little the government can do to assist a consumer who has purchased a vehicle that has insufficient cargo capacity for its intended use." 

-- Edited by FastEagle on Sunday 3rd of January 2016 06:26:57 PM

Cummins12V98 wrote:

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I received the "add 5psi" to the inflation/weight table right from GY Tech Support when I was asking about going from "E" to "G" tires on our Avion 5th wheel.

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Yep, and to make a note, the OP was discussing trailer tires on which all of my comments were based.

Questions about the +5 psi issue. First let me say this. It's not mentioned in any of the approved regulations or industry standards.

When is the +5 psi added?

Is it in addition to the recommended inflation pressure on the tire placard?

Industry standards for RV trailer tires - any design - do not vary for OE tires from the vehicle manufacturer's recommendations.

Industry load inflation charts provide the tire load capacities at various inflation pressures so the target value can be properly set. That target value has been pre-set/pre-determined by the vehicle manufacturer.

More than 90% of all Original Equipment trailer tires have been set at 100% of their maximum load capacity. Therefore there is zero wiggle room for adjustments. Plus sized replacement tires are another subject.

The automotive tire industry is very different from the RV trailer tire industry. When owner's are relying on information from automotive retailers without RV trailer tire experience they are often misled into situations where a trained RV trailer tire retailer would not lead them.

Inflating RV tires of any kind to the load carried is not a product of the Federal Motor Vehicle Safety Standards (FMVSS). It's a product of the trucking industry standards.

A key element in tire safety is the fact that any tire operating 20% below it's recommended inflation pressure is a tire needing an inspection by a trained tire technician. Without a starting point where does the 20% under-inflation start? AT THE VEHICLE MANUFACTURER'S RECOMMENDED INFLATION PRESSURE FOUND ON THE FEDERAL CERTIFICATION LABEL FOR ALL OE TIRES AND LIKE SIZED REPLACEMENTS.

-- Edited by FastEagle on Tuesday 5th of January 2016 06:27:56 PM

-- Edited by FastEagle on Tuesday 5th of January 2016 06:28:31 PM

-- Edited by FastEagle on Tuesday 5th of January 2016 06:29:10 PM

Last Edit: I'm all done with this. I'm starting to repeat things that are easily found with a little research.

The RV Safety & Education Foundation (RVSEF) recommends running trailer tires at maximum sidewall pressure EXCEPT in those rare instances where there is significantly more tire capacity than load being carried.  That recommendation from RVSEF is based on Executive Director, Walter Cannon's relationships with all major tire manufacturers, participation on RVIA safety committees, and involvement with NHTSA as their regulations relate to RVs.  Most of you know that we weighed RVs and did tire and weight safety seminars for RVSEF for four years, but feel free to contact RVSEF directly to confirm.

The reason for the recommendation to run trailer tires at maximum sidewall pressure is twofold.  First, MOST trailer manufacturers are using tires that are barely adequate for a fully loaded trailer, so most of the time a loaded trailer will require maximum PSI in the OEM tires to handle the loads on the tires.  Second, trailer axles are very close together and there is great stress on the tires in tight turning situations; therefore, higher PSI (but no more than sidewall max) reduces the friction which also reduces the skidding and scuffing and stress on the sidewalls of the tires.

So, to answer the original question, most everyone should run trailer tires at maximum sidewall pressure regardless of the inflation tables.  However, more and more larger fifth wheels are coming with higher rated commercial tires that have significantly more load capacity and many of us have upgraded our tires to those higher rated commercial tires (like the Goodyear 17.5-inch H load-rated G114s).  In that limited scenario where the tires have significantly more load capacity (hundreds of pounds) than the actual load they are carrying, it may be beneficial to run the tires at less than maximum sidewall pressure to reduce "bounce" and to get a more "correct" tire patch on the road for braking.

As mentioned, the Goodyear G114s have 4,805 lbs of capacity at a max of 125 psi.  Even those of us with very heavy fifth wheels will generally not come close to 4,805 lbs per tire, so many of us run those particular tires at 110 - 115 psi (perhaps even less) based on the tire manufacturer load inflation tables after having our rigs weighed wheel-by-wheel.

As for the discussion of the "additional 5 psi" in tires over and above the load inflation tables based on actual weights, it comes from the tire manufacturers (again call RVSEF to confirm).  All the major tire manufacturers recommend weighing your RV wheel by wheel to determine actual loads on the tires and then use the load inflation tables to determine the proper inflation pressure for the load the tires are carrying.  However, the tire manufacturers recognize that when you weigh your RV, you are weighing it in a static situation (not moving) whereas there is side to side load shifting as you are going down the road.  So, Michelin for example, recommends using the inflation pressure in the tire load inflation tables that provides an additional 500 pounds of capacity per tire (over and above what the actual weight on the tire is) to provide additional safety margin for shifting weights.  Goodyear has a similar recommendation but it is less than Michelin's 500 pound number.  So, if you look at inflation tables, often to get that extra cushion of load capacity, that often means an "additional 5 psi" which is where that general number comes from (however, sometimes the extra safety margin may require an additional 10 - 15 psi depending on the tire).  The tire load inflation tables are in 5 psi increments, so often (especially with high capacity tires) the carrying capacity in the next block up ("additional 5 psi") in the tables will provide the recommended safety margin.

In David & Cheryl's case, the load inflation table for their Goodyear G614 (G load range, 110 max psi, 3,750 lbs max capacity) shows a carrying capacity of 3,300 lbs at 90 psi.  If the actual load on the tires is in the 3,300 pound range (at least on the heaviest tire on each axle), I personally would run at least 100 psi and more likely 105 - 110 unless I noticed significant "bounce" (and ruled out that the problem is not a suspension issue).  If the actual load is more in the 3,000 pound range or less (running very light), I'd be more inclined to lower the pressure to the 90 - 100 psi range and increase it as load increases over time.

Certainly, overinflation for the load being carried has drawbacks in that the tire runs hard, there isn't as much tire patch on the road which can affect braking and steering (in motorhomes), and there may be less comfort, but underinflation for the load being carried is far worse, so I would tend to err on the side of overinflation (but never over the sidewall max), especially in a trailer where passengers aren't riding.

Howard, thanks for the very detailed explanation!

Our trailer wheel weights were 2975/2850 (left/right) on the front axle, and 3250/2875 on the rear axle. Since I can only set one high/low range on my TPMS, I was going to inflate all the tires to the same PSI based on the heaviest wheel, which was 3,250 lbs. That's where I got 90 PSI from the tables, and I added 5 PSI margin.

However, if I add 500 pounds safety margin per wheel, I would use 3,750 as the weight for the tables. That's the tire's max capacity, so I'd inflate to 110 PSI. 

I have noticed, since I reduced the pressure to 95 PSI a couple weeks ago, that the tires do twist/flex visibly when I'm in a very tight turn, as when maneuvering into a parking spot. That was worrying me. I assume they probably do the same while driving, although not as dramatically, even though I can't see them. For that reason alone, as well as the extra safety margin, I'll probably go ahead and air them back up to between 105 and 110 PSI.