Tire Pressure, Who do I trust?

[aztec7fan]

I have a 1989 Southwind 33-foot Class A and I've learned a lot in the last year about tire weight, pressures and age, thanks to FMCA. I actually weighed the coach for the first time last week and found it was 400 pounds overweight in the back, which I rectified (dumped holding tanks).

The sticker on the inside of the coach from Fleetwood says the pressure in all the tires should be 70 PSI.

When looking at the Michelin inflation tables for the 225/70 R19.5 XRV tires it says the pressure should be around 65 PSI.

Which is accurate/best?

Thanks for any info,

Chris G.

[tbutler]

You have the weight data which has been corrected and "re-weighed"? That with the Michelin charts give you your tire pressure. No doubt the Fleetwood data is for the average vacationer who had much less in their rig than the serious RV'er who will have their rig weighed and used the tire tables to determine proper pressure.

Remember to add 10% to the pressures to allow for the errors in tire gauges. Now there is a real conundrum! Try to have someone calibrate your tire pressure gauges. I haven't found any shop that is set up to do this.

Let's see, if you add 10% to the 65 pounds of the Michelin chart that gives you about 71 pounds. That is close to the Fleetwood recommendations. I would guess that Fleetwood bases their posted pressures on the maximum weight rating of the coach and threw in an extra 10%.

[Guest 2driftrs]

Years ago, the best way to properly inflate a tire (RV, auto, whatever) was to obtain from the manufacturer the centerline (or static loaded radius) height. This is the distance from the pavement to the centerline of the wheel. A lightly loaded tire sits higher than a loaded tire, and adjusting pressures based on the centerline height provided the maximum and ideal running conditions. Goodyear used to post this info on their website, but no more.

I'd be inclined to go by what the coach manufacturer gives, simply because they are basing their pressures knowing what the expected loads will be. Our coach says 80psi on the Goodyear tires, but the Goodyear load chart says we should run 85psi. (We run closer to 80 in the summer and closer to 85 in the winter so our tire pressures at highway speeds are more or less the same year round)

As long as you're within the max pressure for the tire, most tire experts will tell you that to err on the high side is safer than to err on the low side.

[wolfe10]

OK, let's take this from the top:

Tire pressure as posted on the GVWR plaque (coach manufacturer): States correct tire pressure FOR AXLES LOADED TO THE POSTED GAWR. PERIOD. If your actual weight is above or below that weight, the PSI recommendation DOES NOT APPLY. And remember, this is for the OE tires. Newer tires may have different PSI recommendations for that GAWR.

Michelin (and all other tire manufacturer) inflation charts-- state MINIMUM PSI for a given weight. If you have individual wheel position weights, the recommendation is to add 5 PSI to that MINIMUM, based on heavier wheel position on each axle-- all tires on an axle to have the same PSI.

If all you have is axle weights (so you are assuming equal left/right weight distribution), the recommendation is to add 10 PSI to the minimum on the tire chart assuming this does not exceed the tire or wheel maximums.

And, you did well to "redistribute" weight given the overweight axle.

And I agree with Tom, it is a good idea to have your tire pressure gauge calibrated. Most tire stores have high quality pressure gauges on their air systems and will happily let you look at their gauge and then see what your gauge reads on their air system.

Brett

[Guest JamesFauance]

After reading all the reports I guess I am still confused. I have a 2001 Georgetown Class A and had the tires replaced. When I asked the tire dealer what the pressure is he replied "I have to inflate them to the manufactures recommendation which is 105 lbs". I then took the coach to have the front wheels aligned and the RV dealer told me that that was well over the recommended inflation pressure. He showed me the recomended pressure was 80 lbs in the front and 90 lbs in the rear. He said some of the steering problems was caused by over inflation of the front tires. I called the origional tire dealer and told him what was done and he said the manufacturer would not cover the warranty since the pressure is not as recommended.

OK, what am I supposed to do? I must say that the coach did ride very hard when at 105lbs.

[wolfe10]

James,

Welcome to the FMCA Forum.

Go to your tire manufacturer's website and look at their Load/PSI chart for your tires. THEY are the ultimate authority, not a local tire dealer.

he answer is that you hope that your tire dealer is way wrong. If he is correct, it would mean you are loading the tires to their maximum carrying capacity. Said another say, your tires have no safety reserve capacity.

The Load/PSI chart shows the MINIMUM PSI for a given weight. Most recommend adding 5 PSI to that minimum based on heavier wheel position on each axle with all tires on an axle having the same PSI based on heaver wheel position. Add 10 PSI (as long as it does not exceed tire or wheel specs) if all you have is axle weights.

No one can give you accurate advice without knowing the actual weights.

Brett

[marksheinkopf]

I purchased a Itasca Horizon Diesel pusher in February 2010 and had pressure monitor system installed before I drove it of the lot. I had the monitor set to warn me if I am over 10psi from recommended pressure for all 6 tires. It works well and also give the temperature of each tire. It was a $ 500 option but well worth it in my opinion. This way I can spot pressure changes as I drive as long as I keep my eyes on the road and not on the computer display on the dash. I have heard that a tire failure or blowout can cost a tire, but the thousands of dollars of body work damage is much more than the cost of a new tire. Spend the bucks and get a wireless remote tire gauge system professionally installed.

[tomw@johnsondev.com]

I have a 43 foot MH that has a loaded weight of 20k on the rear and 12k on the front. I assume that the 12k on the front will be split for the chart but with a single tag on the rear is the 10k per side split by 2 or 3???

[wolfe10]

Different tire manufacturers style their PSI/load charts differently-- you will need to look at your tire manufacturer's chart and its description.

If it is not clear to you, post what tire brand and size you have.

And just like the front axle (two tires) the tag axle would be treated the same. BUT, remember, tire pressure is based on ACTUAL weight, not GAWR.

It is very unusual for a tag axle to carry its full GAWR!

Brett

[ldmiller34@yahoo.com]

I'd be interested in comments relative to pressure changes with altitude. We recently returned to the Seattle area from a 2 week excursion to the Colorado Plateau area. Tire pressure was set at 65 psi (cold) on all 6 of my tires per 'Coach House' recommendations. (Weight is pretty evenly distributed in this rig.) Of course this was at near Sea Level conditions. In checking cold pressure at much higher altitudes (many places on the Colorado Plateau are near 9000'). I found it had risen over 10 psi. I resisted the temptation to adjust them as they were still below the max limit set by Michelin. In checking after we got home I noted that they were back to the 65 psi set at departure. Was I right to leave them alone on the trip?

[wolfe10]

The PSI increases .48 psi for every 1,000 feet increase in altitude. So, PSI increase at 9,000' due to altitude would be around 4.5 PSI.

Tire inflation PSI increases 2% with a temperature increases 10° F.

With the cooler temperatures at altitude, it is quite possible that PSI will decrease at high elevations.

Brett

[campnow2008]

We have a 2001 35 foot Aerbus. We had it weighted by RVSEF at an FMCA rally so we know what each wheel weight is and we are right at the max gross of 20,500 lb. So I use the recommended 80 psi for the 245/70 R19.5 Goodyear tires. I bought two pressure gages from Camping World and they disagreed by 5 to 7 psi, so I bought a digital gage (Slime 20071, reviewed in the Aug 2010 Motorhome magazine) believing it would be more accurate. Since it agrees with one of the other gages, those are the two I use to check pressures. At 80 psi, we have excess capacity, so I saw no reason to inflate the tires to the max of 85 psi. We ran this way for 6 years and 27,000 miles until this past July 3rd when the RH front tire let go with a Ka-Blam! It was a warm sunny day and we were 3 hours from home ending a 2200 mile trip to the Grand Tetons and Yellowstone. Fortunately, instead of the narrow, two lane, no shoulder mountain roads we had been on for a month, we were on I-90, west of Moses Lake, Wa. on a straight, wide stretch of freeway. Good Sam ERS and in 1 1/2 hours we were back on the road. The tire had a golf ball sized hole in the inside wall, but the tread didn't separate and no damage was done to the coach. The tires looked good, no cracks, no checks, no evidence of impending failure and lots of tread left, but they are 10 years old. Next week we will be sitting on 6 new Goodyears. A friend of mine with a 1999 33 foot Southwind experienced the same type of failure in tires of similar age. Now I'm a believer that it is age, not mileage that determines when motorhome tires should be replaced.

[jcmace]

If you don't know the weights at each wheel, you're guessing at best. Until you've weighed each wheel to be safe you should set them for the tire manufacturer's maximum recommended pressure. Yes, the ride may be rougher. But an under inflated tire will let go with possibly catastrophic effects. At the very least, you must weigh each axle with motorhome loaded as you would normally travel.

You should be using the tire manufacturers charts to properly set your pressures and nothing else.

You should be replacing your tires at the tire manufacturers recommended intervals- no matter what the mileage. Michelin has recently gone to a 10 year interval, but used to recommend every 7 years. A couple thousand dollars every several years is dirt cheap compared to what a blow out can do.

[aztec7fan]

I think I'll go with 70 PSI since this is reccommended by Fleetwood for the coach and it's 5psi higher than on the reccomendation chart, which should be acceptable.

A little history: I did own a 34' 1991 Tiffen Allegro Bay which is very similar to my current coach about 10 years ago. BUT, I did everything wrong and paid for it. I stored the coach on a dirt/mud pad for months at a time, with the tires not covered from the sun. I only checked the pressure once before a long trip and NEVER weighed the coach. I never even knew about tire ages and so they were probably around 10+ years old. I also drove at the posed speed limits, including 75mph.

Because of this ignorance, I had 4 tires fail in 3 years, most were where the tread seperated from the tire, and one blowout. Thank goodness I never had a situation where the coach was damaged or someone was injured, but I spent too much time stranded on the side of the road, waiting for assistance, never again!!!!

Thank you for educating me! I learned everything above was wrong in just this last year of belonging to FMCA and reading articles on FMCA.com.

[riggarob]

FYI, you can buy a calibrated tire guage from NAPA for about 30 bucks. I never leave home w/o it! Robbie

[F371199]

Weigh your rig, each corner, every year (our's seems to gain weight on occasion and has to be put on a diet).

Inflate your tires according to these weights and to the specs of the manufacturer of the tires you currently have installed (better on the high side than too low).

Check your tire pressures before each, and every, driving stint with a tire pressure gauge which is accurate. A good TPMS not only provides constant monitoring of your tire pressures but is most-often more accurate than your standard, run-of-the-mill analog stick gauge.

Unlike long-haul semi-tractor (both steer and drive axles) and trailer tires... unless you're routinely putting on 100K-150K miles a year on your rig, your "you name the brand" tires are more than likely going to succumb to old age (learn how to read the DOT code on your mfg's tires) long before the tread is gone. $3,000 every 5-7 years translates into $428 - $500 per year for "tires". Compare that with your initial investment cost, with your, already, routine, $1K - $3K annual service & maintenance costs, your "you name the figure" annual operational costs, plus the headaches (and heartaches, and add'l costs, perhaps worse than that even!) that come with an unexpected tire failure, and you'll hopefully see "new tires every 5 - 7 years" as a good/prudent investment.

[tireman9]

We have a 2001 35 foot Aerbus. We had it weighted by RVSEF at an FMCA rally so we know what each wheel weight is and we are right at the max gross of 20,500 lb. So I use the recommended 80 psi for the 245/70 R19.5 Goodyear tires. I bought two pressure gages from Camping World and they disagreed by 5 to 7 psi, so I bought a digital gage (Slime 20071, reviewed in the Aug 2010 Motorhome magazine) believing it would be more accurate. Since it agrees with one of the other gages, those are the two I use to check pressures. At 80 psi, we have excess capacity, so I saw no reason to inflate the tires to the max of 85 psi. We ran this way for 6 years and 27,000 miles until this past July 3rd when the RH front tire let go with a Ka-Blam! It was a warm sunny day and we were 3 hours from home ending a 2200 mile trip to the Grand Tetons and Yellowstone. Fortunately, instead of the narrow, two lane, no shoulder mountain roads we had been on for a month, we were on I-90, west of Moses Lake, Wa. on a straight, wide stretch of freeway. Good Sam ERS and in 1 1/2 hours we were back on the road. The tire had a golf ball sized hole in the inside wall, but the tread didn't separate and no damage was done to the coach. The tires looked good, no cracks, no checks, no evidence of impending failure and lots of tread left, but they are 10 years old. Next week we will be sitting on 6 new Goodyears. A friend of mine with a 1999 33 foot Southwind experienced the same type of failure in tires of similar age. Now I'm a believer that it is age, not mileage that determines when motorhome tires should be replaced.

By any chance do you know if the sidewall cord is Steel or Polyester? This information is posted on the sidewall. I did specialize in failed tire inspection and Root Cause analysis. While a single picture is not really enough to go on there is some visible evidence that suggests the possibility of an impact cut. BUT I would need to see the rest og hte tire and it really isn't worth the effort to shio the tire to Akron.

If you still have the tire having a few more well lit pictures might provide more clues.

[tireman9]

Some comments from a retired tire design engineer.

Some important information is missing from the various posts. When posting questions about load and inflation capabilities for tires you need to include the “Load Range†This is a letter code such as D, E, F etc with most RVs using something between E and G. Some class A MH might go up to H and some class B may be down at C or D.

Also be sure to give the complete size such as LT235/70R16 or 265/70R22.5. Saying you have a 17†tire or a 265 doesn’t provide enough information for us to really help answer your question.

Regarding Load & Inflation tables. With only a few exceptions for some individual tire designs and sizes, all major tire companies use the same tables for showing the inflation so until you find the tables for your particular size you will not be too far off. Usually the exceptions are for additional load capability of a few hundred pounds or an extra 5 or 10 psi for RV application.

The Load and inflation information molded into the tire should never be exceeded. These are Maximum load at the stated inflation.

RV manufacturers can only guess at how much “stuff†you carry around. What they are giving you on your load charts are the Maximum loads based on the design of the various components used by the manufacturer such as axles, springs, wheels and tires. This is why it is important that you know the real weight for each corner of your RV when fully loaded. If having your holding tanks full results in higher load on your tires then simply dumping your tanks is only a temporary “fixâ€. Temporarily unloading the tires will not repair the damage you have done to the tire anymore than putting air back in a tire after you ran it underinflated. Damage is cumulative and cannot be reversed. You must find a permanent solution to prevent operating your RV when any component in overloaded.

RE Tire gage accuracy. If you select a digital gage and compare it to another gage and they both read the same +/- 2 psi or less I think you can count on that gage being sufficiently accurate.

RE a 10% cushion on selecting inflation. It is a good rule of thumb to have a “cushion†but must be applied correctly. This does NOT mean you should increase the inflation above the MAXIMUM inflation molded into the tire. For example: If we had a 225/70R19.5 Load Range F tire we can see the markings on the sidewall indicating it is rated at 3640# max when inflated to 95 Psi and sunning in a single application or 3415# as a dual. If you weigh tour RV and find that the heavier side of the rear (dual) weighs 6100#. According to load tables we can see that this tire is rated at 3,000# dual at 85 psi which is not enough so we need to go to the next higher inflation or 90 Psi. This is your MINIMUM recommended inflation. You could simply increase this minimum by 5 psi and have a nice cushion. However if we found the heavy side of the rear really weighed 6,800# we would need to inflate our tires to 95 psi Minimum. Since this is also the maximum inflation the tire is rated for you cannot increase your inflation to 100 psi. In this case your only option is to unload your RV.

What does your RV really weigh?

A simple Front axle and rear axle weight can be misleading as I have seen reports of one side being loaded by 3% to 7% more than the other side. You really need to get each corner weighed. You can get this done at some FMCA Rallies or check http://www.rvsafety.com/ and under RV Weighing Program check their Schedule.