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Old 03-04-2021, 07:47 PM   #1
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Am I reading my tire pressure chart correctly?

Appreciate any help here. Want to make sure I understand the CAT scale results and the inflation guide for my tires. Please let me know if I made a mistake or something looks wrong.
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Old 03-04-2021, 08:38 PM   #2
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Are you sure that is the correct chart for the duals? For example, my Michelin XRVs show:

3,980 for a single tire at 90 psi

but

7,510 for a dual pair at 90 psi

or noticeably less than double the single tire rating. That happens for all tire sizes, not just mine.

Also note that you must be certain that "as weighed" is the absolute maximum your coach will ever weigh. Otherwise I'd be going up a notch or two.

In fact, I carry 10% over the load chart to accommodate for changes in temps. I'm supposed to be 90 all the way around and I usually air up to 98 - 100. When I left Florida last March I aired up to 100 psi in 80 degree temps and when I got home to 40 degree temps my tires were about 92 psi the next morning so I should have gone a bit higher.

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Old 03-04-2021, 09:08 PM   #3
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I'm relatively new to all this and learning all the time. I had a tire pressure lesson the other day that I hadn't considered at the time. With the record cold snap we had here in Texas two weeks ago, I had set the pressures in my tires before we left. I run 100 in the steer tires and 95 in the back. It was about 19 degrees outside when I set the tire pressures. We got to Terlingua in the Big Bend area and the temps were now about 80 outside, and my steer tires were up at 111 and the backs were up around 105-106. I knew temperature made a change in tire pressure but I didn't realize it was that much. Going up in ambient temperature isn't a tremedous big deal unless you're set at very high pressures set on a very cold day. However setting them on a hot day at relatively low presure and then driving into much colder areas can obviously cause a serious problem and perhaps a tire failure. Lesson learned.
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Old 03-04-2021, 09:24 PM   #4
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Quote:
Originally Posted by NXR View Post
Are you sure that is the correct chart for the duals? For example, my Michelin XRVs show:

3,980 for a single tire at 90 psi

but

7,510 for a dual pair at 90 psi

or noticeably less than double the single tire rating. That happens for all tire sizes, not just mine.

Also note that you must be certain that "as weighed" is the absolute maximum your coach will ever weigh. Otherwise I'd be going up a notch or two.

In fact, I carry 10% over the load chart to accommodate for changes in temps. I'm supposed to be 90 all the way around and I usually air up to 98 - 100. When I left Florida last March I aired up to 100 psi in 80 degree temps and when I got home to 40 degree temps my tires were about 92 psi the next morning so I should have gone a bit higher.

Ray
I agree, running strictly by a load/inflation chart means your tires are operating at 100% capacity; ANY pressure drop means the tires are under-inflated. Tireman9 recommends adding 15% over load/inflation chart listing.
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Old 03-04-2021, 09:37 PM   #5
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Quote:
Originally Posted by NXR View Post

Also note that you must be certain that "as weighed" is the absolute maximum your coach will ever weigh. Otherwise I'd be going up a notch or two.


Ray
I disagree with this statement. I weighed my coach as it is typically loaded for travel and set my tire pressures appropriately. Sometime in the future I may load it differently for an extended trip. Knowing I have added more weight I will weigh it again and adjust my pressures. But in the meantime why would I run higher pressures to match the absolute maximum it might ever weigh?
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Old 03-04-2021, 10:05 PM   #6
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Quote:
Originally Posted by Doug427 View Post
Going up in ambient temperature isn't a tremedous big deal unless you're set at very high pressures set on a very cold day. However setting them on a hot day at relatively low presure and then driving into much colder areas can obviously cause a serious problem and perhaps a tire failure. Lesson learned.

It's normal for pressures to fluctuate with changes in temps. It's physics. Tires are designed with this fact in mind. That's why it's SOP to recommend not checking tires during a run, and NEVER adjust the pressure on a tire that's heated as it's been driven- rather check the tires cold, before they've been driven. Here's a table that gives pressures compensated for ambient air. I've noted during our 11,000 mile loop last summer that they seem to be pretty accurate.
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Old 03-04-2021, 10:18 PM   #7
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Originally Posted by Bsipe01 View Post
It's normal for pressures to fluctuate with changes in temps. It's physics. Tires are designed with this fact in mind. That's why it's SOP to recommend not checking tires during a run, and NEVER adjust the pressure on a tire that's heated as it's been driven- rather check the tires cold, before they've been driven. Here's a table that gives pressures compensated for ambient air. I've noted during our 11,000 mile loop last summer that they seem to be pretty accurate.
Hey, I like that PDF file it’s very accurate
My pressures are based on a CAT scale weight of my MH with a full tank of fuel. Cat calculated pressure are about 100 all around.
I add 5 lbs just in case I add some weight also I didn’t weigh with my Jeep so I added it to the calculations
I run 105 all around in summer, cold winter I start out about 99.
I don’t add, I just drive, pressure goes right up, temps stay cool.
Drove from Pa to Fl at Christmas, left home with about 99-100, 20 minutes on the highway I was up to 105-108 temps were 30 F
In Florida it was 105 all around. Didn’t add or subtract.
It’s the years of doing this stuff, you learn over time.
If I added according to the PDF in summer to 110, my tires would be running 140 on the road, not safe. I'll keep it at 105 summer 65-90 degrees. I don’t like pressures over 135 in summer heat and tire temps at 90-100
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Old 03-05-2021, 08:02 AM   #8
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I disagree with this statement. I weighed my coach as it is typically loaded for travel and set my tire pressures appropriately. Sometime in the future I may load it differently for an extended trip. Knowing I have added more weight I will weigh it again and adjust my pressures. But in the meantime why would I run higher pressures to match the absolute maximum it might ever weigh?
You, maybe not. The OP? Maybe. The OP did not specify the loading condition when he/she weighed the coach.

It could have been weighed fully loaded for a long trip or weighed completely empty of everything, having just come out of winter storage. The OP could tow a vehicle on a dolly or trailer, which was not connected for the weighing, which could add 400 pounds of tongue weight or more.

"Fully loaded" versus "empty of everything" can be a difference of multiple thousands of pounds. It's about 2,500 pounds for ours, most of it on the rear axle.

By "maximum your coach will ever weigh" I did mean "fully loaded for a trip" and not "its GVWR" because those two figures may be thousands of pounds apart for a DP. For a gasser, probably not.

I put air in my tires twice a year, Spring and Fall. Perhaps other people like airing up their tires more often. I do not. That's why I use a TPMS; to avoid having to mess with the tires.

The OP's Tiffin appears to have a GVWR of 51,300 pounds while their CAT Scale weights total to 42,575, a difference of 8,725 pounds. Never having owned a DP I have no idea what that weight margin represents in terms of loading. Nor do I know the GAWR numbers for the three axles of that Tiffin.

I always try to write my answers so they will not unintentionally mislead future readers looking for general information. The OP's initial request did not state how it was loaded and a new motorhome owner searching for similar information might not know how loading can affect weights and thus tire pressure chart interpretation.

Hope this help clarify what I was thinking.

Ray
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Old 03-05-2021, 08:30 AM   #9
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Hey, I like that PDF file itís very accurate.
The rule of thumb is 2 PSI of change for every 10 degrees F of change. That chart tracks that very closely and that's close enough for me since a tire pressure gauge usually has a +/- 3 psi accuracy tolerance. My TST 507 TPMS cap sensor manual claims +/- 3 psi accuracy.

Quote:
also I didnít weigh with my Jeep so I added it to the calculations
Would you please clarify that statement?

If you are flat towing then the only weight you should need to add, if any, is the weight of any drop hitch plus one-half of the weight of the tow bar. That's usually well under 100 pounds added to the rear axle of the motorhome.

If towing with a trailer or with a dolly, then yes, you should add in the tongue weight but not the weight of the vehicle itself.

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Old 03-05-2021, 08:48 AM   #10
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I'm a little surprised at the weight distribution is close to the old 60/40 (drive to tag) that Tiffin had years ago with the proportioning valves used then. After several of us brought it to their attention, they eliminated the prop valves and due to air bag sizes, ended up with a 70/30 distribution ratio. That took weight off of the steer and tag and increased the drive up to near 20,000# for better traction on rain slick or slippery conditions. I along with several other tag owners installed manual regulators to set the ratio for better traction and weight distribution. Tiffin agreed that 60/40 wasn't good at that time. I have to wonder what changed their mind.
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Old 03-05-2021, 09:05 AM   #11
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I select the reading on the chart that is next weight above the actual weight needed, and add 10 PSI.
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Old 03-05-2021, 09:44 AM   #12
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How to properly inflate your motorhome tires

Quote:
Originally Posted by NXR View Post
You, maybe not. The OP? Maybe. The OP did not specify the loading condition when he/she weighed the coach.

It could have been weighed fully loaded for a long trip or weighed completely empty of everything, having just come out of winter storage. The OP could tow a vehicle on a dolly or trailer, which was not connected for the weighing, which could add 400 pounds of tongue weight or more.

"Fully loaded" versus "empty of everything" can be a difference of multiple thousands of pounds. It's about 2,500 pounds for ours, most of it on the rear axle.

By "maximum your coach will ever weigh" I did mean "fully loaded for a trip" and not "its GVWR" because those two figures may be thousands of pounds apart for a DP. For a gasser, probably not.

I put air in my tires twice a year, Spring and Fall. Perhaps other people like airing up their tires more often. I do not. That's why I use a TPMS; to avoid having to mess with the tires.

The OP's Tiffin appears to have a GVWR of 51,300 pounds while their CAT Scale weights total to 42,575, a difference of 8,725 pounds. Never having owned a DP I have no idea what that weight margin represents in terms of loading. Nor do I know the GAWR numbers for the three axles of that Tiffin.

I always try to write my answers so they will not unintentionally mislead future readers looking for general information. The OP's initial request did not state how it was loaded and a new motorhome owner searching for similar information might not know how loading can affect weights and thus tire pressure chart interpretation.

Hope this help clarify what I was thinking.

Ray
X2.

To the OP: I found a good video on YT that explains this process perfectly if you want.

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Old 03-05-2021, 11:11 AM   #13
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Quote:
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Appreciate any help here. Want to make sure I understand the CAT scale results and the inflation guide for my tires. Please let me know if I made a mistake or something looks wrong.
On the front, you're a smidge over the 95 psi weight, so I would go to the 100 psi for safety. Yours is like mine on the rear and tag, I don't meet the minimum weight. In that case, I would run the 85 psi in the duals and tag.

Once you have driven for a bit, take a good look at the tires, especially the fronts and see how they're wearing. Overinflated will be center wear, underinflated will be both outside edges wearing faster. I like to run as close to the minimum weight in the front for ride comfort.
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Old 03-05-2021, 11:12 AM   #14
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Thanks for the replies. I should have mentioned that we had just filled up with gas and the three tanks were filled roughly 50%. We full time so we were fully loaded with our stuff.

Iím going to read more about it and I appreciate the useful links.
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