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Old 12-09-2020, 11:17 AM   #1
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Proper Tire Pressure

Have a question regarding “best” tire pressures for improving ride. I took the RV to local Cat Scale for front/rear axles weights with empty Fresh water, gray and black tanks.

Front axle = 7340lbs
Rear Axle = 13400lbs

Looking at the Michelin pressure charts for my tires 235/80R22.5 it recommends 80 psi on all tires with current weights. Should I add 5 psi and go with 85psi to compensate for added cargo on trips?

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Old 12-09-2020, 11:30 AM   #2
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Quote:
Originally Posted by DiverBob57 View Post
Have a question regarding “best” tire pressures for improving ride. I took the RV to local Cat Scale for front/rear axles weights with empty Fresh water, gray and black tanks.

Front axle = 7340lbs
Rear Axle = 13400lbs

Looking at the Michelin pressure charts for my tires 235/80R22.5 it recommends 80 psi on all tires with current weights. Should I add 5 psi and go with 85psi to compensate for added cargo on trips?

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Attachment 310760
105% of front axle weight is 7707 lb, which is really close to the 80 psi value. 105% of the rear is 14,070 lb, which lines up very closely with the 80 psi value again. BUT, you're not loaded, and 5% (or more) is reasonable for side-to-side discrepancies. If it were me, I'd air it up to the door post pressures, load it for travel, then weigh it and adjust from there, after waiting for the tires to cool overnight again.
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Old 12-09-2020, 11:32 AM   #3
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Do you not take fresh water with you? IMO you should take some just in case the water is off, or repugnant where you end up. Was the mh packed as you go camping with clothes food, toys, people? If not you need to add some weights to those from the scales. I’d start with the 85. As an aside what does the placard numbers show for PSI?
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Old 12-09-2020, 11:44 AM   #4
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My opinion when unable to get actual 4 corner wts is do axle wts with...
Full fuel (& DEF if applicable)
Full fresh water
Fully loaded gear for a trip including food & drinks / fridge
Normal driver & passenger(s)
Empty holding tanks

Then allow 10% from avg for side to side variation
Find table PSI and add 5 PSI as a safety factor

Best to do 4 corners if / when possible and try to correct actual side to side variations and front to rear heavy / light loads vs axle capacities.
I was surprised to find about 10% side to side difference and found factory build wts confirmed a large portion of the variation was factory build.
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Old 12-09-2020, 04:51 PM   #5
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Originally Posted by wildtoad View Post
Do you not take fresh water with you?
We do take a full fresh water tank with us when we travel. I winterized our coach last week so all tanks are drained except for minute amount of pink stuff in the gray and black tanks.

A Full freshwater tanks adds 867lbs for us over the rear axle. (104 gallons X 8.34lbs per gallon)

Going to try 85lbs all the way around to start since just me and DW. If we have additional passengers on a trip, will take them to 90 psi.

Thanks for the response folks.
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Old 12-09-2020, 06:44 PM   #6
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This raises another question

My 4 corner actual weights compared to GVWR compared to the Tire Guide are so far apart things make less sense than after reading hours of threads and posts on this subject so I’m hoping for some help.

Cold/hot weather, altitudes, hp and trans aside, I’m at 90psi on all 6 as recommended by the dealer at delivery.

Fleetwood delivery weight new was 25,600lbs
Steer Axel GVWR is 12,000
Drive Axel GVWR is 20,000
Michelin X Energy Z 275/80R22.5 LRH Single max 7160lbs pressure at 120psi

Weights came in with 50% cargo, fuel and water capacity
Steer D 4,850. Steer P 4,650 TTL 9,500
Drive D 9,500. Drive P 9,300 TTL 18,800

If I follow the Michelin Tire guide, psi should be 75lbs on the front, 80lbs in the rear. This doesn’t sound right.
Should I be using the guide, is 90lbs even in the vacinity?
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Old 12-09-2020, 07:11 PM   #7
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Quote:
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Weights came in with 50% cargo, fuel and water capacity
Steer D 4,850. Steer P 4,650 TTL 9,500
Drive D 9,500. Drive P 9,300 TTL 18,800

If I follow the Michelin Tire guide, psi should be 75lbs on the front, 80lbs in the rear. This doesn’t sound right.
Should I be using the guide, is 90lbs even in the vacinity?
If you add 5% on each actual axle weight planning for additional cargo, your weights put your steer axle at 85psi and your drive axle at 90psi. You are probably good to go with 90psi all around if your coach rides and steers well at that pressure.

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Old 12-09-2020, 07:32 PM   #8
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Front may be ok at 85 psi per tire. On the rear though, adding just the water over it puts the weight over 14,200 lbs. I can imagine it being fairly easy to go over with additional supplies and other incidentals. I'd opt to run higher, at least 90 psi all the way around to make sure tire capacity was covered in all situtations.
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Old 12-09-2020, 08:57 PM   #9
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Instead of guesstimating, why not do it right? When coming out of storage, load it as though you were going on a month long trip. Full fuel, full fresh water, food clothing, tools misc supplies two adults and anything else you might want to have along. Do a weight that will be the maximum it will ever weigh. Then add whatever safety margin you want. The result will get you the correct and safe tire pressures.
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Old 12-10-2020, 05:47 AM   #10
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[emoji106] What Crasher said!
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Old 12-10-2020, 06:09 AM   #11
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[emoji106] What Crasher said!

What Winemaker2 said.

Remember, the sticker tire pressures are supposed to be the minimum to support the GAWR (gross axle weight rating).

My sticker is 90 all the way around. When loaded as Crasher said, with a car in tow, we were within 600 pounds of our max weights on the rear axle (but we normally carry 1/2 a tank of fresh water so I have some margin there).

So I decided to run at 90 plus abut 10% on all tires to compensate for temperature variations during the mornings. Last winter we left Florida in 80 degree temps with the tires at 100 PSI each. We arrived in northern Ohio with temps in the high 30's and the tires were at 91 PSI.

I think having a TPMS is even more critical than precise tire pressures.

Ray
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Old 12-10-2020, 11:15 AM   #12
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What Crasher and Winemaker said!

I thought about that when typing my first post but neglected to mention it. I will now. Imo, it makes sense to load everything up first (fresh water, food, clothing, supplies) and then get the rig weighed. That way you'd know what you had (no guessing) and could use the tire chart to get minimum psi. You could then add the 5 or more percent if you wanted for that extra margin of coverage..

Edited to add: Depending on where the fresh water tank is and how full it is can make front axle weight vary. Another reason to have things loaded up first before weighing.
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Old 12-10-2020, 05:01 PM   #13
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Great guidance, thank you!

So, if I max out the GVW limits and then add 5%, I’m right at reaching the Tire Guide For 90psi for the rear (dual), 100 psi for the the front (single).

Does it matter if you run these pressures if we’re taking a few short, less than 500 mile round trips at 2000-4000lbs under GVW? Being over-inflated is more about sacrificing ride versus under-inflated which can be dangerous for a number of reasons, correct?

Since we do a number of short lighter weighted trips, is it necessary that we readjust these pressures up and own or should I just stick with the top end psi,s??

Again, you are really clearing the air (Forgive the pun) on this for me.

Thank you
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Old 12-10-2020, 08:34 PM   #14
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The Goodyear RV tire inflation and loading pamphlet says this about lowering pressure to obtain a softer ride:
"
Tire pressure should never be reduced below the vehicle manufacturer's recommended levels to support load conditions in order to improve the ride quality of a vehicle. The difference in ride quality is not significant. When minimum inflation pressure requirements are not met, tire durability and optimum operation can be affected."
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