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Old 11-15-2015, 08:21 AM   #1
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275 70R22.5 tire pressure Holiday Rambler

I have 275 70R22.5 Firestone tires. What tire pressure do you run? the max pressure recommended is 130 lbs. and with that this is the max load capacity. someone told me they only run 100 pounds but that really lowers the load capacity according to what I can find. thoughts? thank you in advance
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Old 11-15-2015, 08:36 AM   #2
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Inflating all to 100 psi or more will give you a harsh ride. The best combination of safe load bearing and comfortable ride will be had if you inflate to the Firestone recommended inflation for the load you're carrying.

First, get your coach weighed in travel configuration. Individual corner weights are preferable, but at least axle weights. Then go online to the Firestone website and download their inflation tables. This will tell you the minimum inflation for each tire, based on the actual weight it is carrying.

There have been dozens of conversations about this on this Forum, available for reference simply by searching.

Good Luck!


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Old 11-15-2015, 08:43 AM   #3
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Anyone that tells you what to run your tires at WITHOUT knowing the weight of your coach is looking to get you killed! It's as simple as that! Without knowing the weight, how would you know if you're under inflated, thus risking a blowout?
Get it weighed, get the chart, and get it right! (Unless your life is not worth it.)

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Old 11-15-2015, 10:29 AM   #4
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You need to weigh the coach.

Until you do, I would

- get the load inflation table for your specific model of tire (download from Firestone's website)
- find the Gross Axle Weight Ratings on your coach data plate
- determine the minimum pressure required to carry the GAWR from the load inflation table
- add 10% to account for unequal side-to-side weight distribution, and inflate your tires to that number

If you're not sure that you're doing it right, post the above information and plenty of folks will be happy to assist.



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Old 11-15-2015, 11:23 AM   #5
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I can give you a pretty good idea what to run, since I had the same coach. I had 2005 Diplomat 36SKQ for 9 years. A good starting point would be 100 psi in the front and 85 psi in the rear duals.
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Old 11-15-2015, 02:22 PM   #6
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Googled your sises and found 148/145 M wich means for single load 6945lbs AT your given 130 psi, and for dual load 6393lbs AT 130 psi a tire.

Made extra save pressure loadcapacity list with spreadsheet I made.
Will try to copy it here
First per tire
PSI 75 80 85 90 95 100 105 110 115 120 125 130
S 4190 4445 4695 4950 5200 5450 5700 5950 6200 6445 6695 6945
D 3850 4085 4320 4550 4785 5015 5245 5475 5705 5935 6160 6390

And below for per axle so single load 2 tires on the axle and dual load 4 tires on the axle.

PSI 75 80 85 90 95 100 105 110 115 120 125 130
SINGLE 8380 8890 9395 9900 10400 10905 11405 11900 12400 12895 13395 13890
DUAL 15415 16350 17280 18210 19140 20065 20985 21905 22825 23740 24655 25570


If you compare these lists to those given by the tire makers , you will see lower loadcapacity for the same pressure , and to my conclusion my lists are close to laws of nature, and those of tire american tire makers give to high loadcapacity in the lower pressures so give to much deflection so heatproduction.

But someone already wrote to add 10% to the weighed loads.
If that leads to higher loads then given for 130 psi , then know this is not the maximum alowed cold pressure of the tire. Most American tire makers allow for these in fact trucktires 20 psi above that so 150 psi.

And then max speed is M speedrated 130km/81m/h, so bumping border is far away then . I determined that for if real load is below 85% of calculated loadcapacity for 160km/99m/h ( then lesser deflection).
This would give a Loadindex of 3 lower so 145/141.
Then for single load you can use the dual load of top list.

But first determine the loads per tire or second best per axle, but as long as you did not do that use the GAWR's and I asume front to be single load and rear to be dual load

This all gives a range in the pressure at wich your tires are save and gripp and comfort is still acceptable.
Then fill to highest pressure that wont give bumping and if in time it drops to pressure belonging to 81m/h with 10% reserve for unequall load R/L , then your tires still wont damage by to much heatproduction.
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Old 11-15-2015, 05:44 PM   #7
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4 corner weights is the right way as stated above.

I have a similar coach for example I run 85 rear & 95 front, but that based on getting the weights.

When I first bought it the PO had 110 frt 100 rear, all wrong

Just go to truck stop pull one side on one side off
Then again for the other side

YOu go by the front side that is heavier like wise for the rear the heavier side dictates which chart weight to go by.

By the way after I corrected the air pressure it made a noticeable difference in the ride.

After all that is done check your ride height......that's another story
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Old 11-15-2015, 07:42 PM   #8
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OK based on the gross weight (12k front and 20K rear) look at it I would need 110 in fronts and 100 in rears if i read the canrt right..lines in it has shifted though thanks for a starting point going to get it weighed when I can... thx ps Firestone FS561's
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Old 11-16-2015, 05:31 AM   #9
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Could not edit anymore so quoted and tried to put the pressures more above the loads. Took a whole lot of previews to get it like this.
But dont forget to add some reserve first . Dual loads as you have behind already are calculated for lesser deflection so the 85% at wich bumping begins is given from single load deflection .
Tried to make a list with other spreadsheet for ST tires , but discovered a little failure in setting over to tire from axle, in it so stopped.

But adding first 10% would give ,looking back in the lists for 12000x1.1=13200/125psi and for rear 20000x1.1=22000= about 110 psi.
Mostly these heavy motorhomes are not filled to their GAWR's but never say never.
And most weight mostly on rear and front axle not loaded to the GAWR of yours 12000lbs. But this I concluded from the smaller motorhomes , so can be off.
Weighing fully loaded with persons and liquids is the only way to be shure.
If you ever do that give them in this topic , so we can see if I am right in this case.
Front mostly stays below GAWR and rear will get to it or even overloaded for the smaller motorhomes.

Quote:
Originally Posted by jadatis View Post
First per tire
PSI 75 / 80 / 85 / 90 / 95 / 100 / 105 / 110 / 115 / 120 / 125 /130
S 4190 4445 4695 4950 5200 5450 5700 5950 6200 6445 6695 6945
D 3850 4085 4320 4550 4785 5015 5245 5475 5705 5935 6160 6390

And below for per axle so single load 2 tires on the axle and dual load 4 tires on the axle.

PSI........ 75 / .. 80 / ..85 / .. 90 / .. 95 / ..100 / 105 / 110 / 115 / 120 / 125 / 130
SINGLE 8380. 8890. 9395 . 9900.10400 .10905.11405.11900.12400 12895 13395 13890
DUAL ..15415 16350 17280 18210 19140 20065 20985 21905 22825 23740 24655 25570

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Old 11-16-2015, 03:20 PM   #10
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Interesting they spec the 275's on his '03, I see it is a different chassis on mine they are spec'd for 255's

275 are 10.8 wide
255 are 10.4 wide

I was thinking about using the 265 or 275's on my fronts next time around not really sure it will make a tiny difference in front stability or not.

----

Now back to tire pressure & weights.
His coach is a 36' mine is 38 I assume mine is tad bit heavier.

My weights are
both f/r 8140/15080
left side 4700/7160
Rgt side 3540/7660

I run 85 rears and 95 front, ran about 10-12,000 miles to date feels fine.

So although the max is 28-30,000 the coach is no where near it.

The differential L/R was corrected by several hundred pounds after ride height adjustments.
Now they are much closer, can't find the scale sheets right now.


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Old 11-16-2015, 10:31 PM   #11
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Quote:
Originally Posted by grthigpen View Post
I have 275 70R22.5 Firestone tires. What tire pressure do you run? the max pressure recommended is 130 lbs. and with that this is the max load capacity. someone told me they only run 100 pounds but that really lowers the load capacity according to what I can find. thoughts? thank you in advance
Near the drivers area you'll find a tire sticker listing the recommended tire pressure. This is what Holiday Rambler recommends based on their research.
Those tire inflation charts list the MINIMUM air pressure for the corresponding load, not the optimum.
The major tire manufacturers recommend following the vehicle mfgrs recommended tire pressure instead of following the load/inflation charts; as does the Rubber Manufacturers Association.( ch 4,pg 51) which states:
However, never use inflation pressure lower than
specified by the vehicle tire placard, certification
label or owner’s manual. Nor should inflation
pressure exceed the maximum pressure molded on
the tire sidewall
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Old 11-17-2015, 09:34 AM   #12
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Quote:
Originally Posted by jimbo2013 View Post
Interesting they spec the 275's on his '03, I see it is a different chassis on mine they are spec'd for 255's


Now back to tire pressure & weights.
His coach is a 36' mine is 38 I assume mine is tad bit heavier.

My weights are
both f/r 8140/15080
left side 4700/7160
Rgt side 3540/7660

I run 85 rears and 95 front, ran about 10-12,000 miles to date feels fine.

So although the max is 28-30,000 the coach is no where near it.

The differential L/R was corrected by several hundred pounds after ride height adjustments.
Now they are much closer, can't find the scale sheets right now.


.
You most always see this crossed weightdifferences R/L on front and rear axles. calculated weightdivision front L/R 57.7/42.3 so enormous.
Rear 47.5/52.5 often weighed.

But do you mean that this only got less by ride height adjustment?
My conclusion once was that by leveling the car , the torsion forces are brought back to zero so even more weightdifference R/L.
But this then was leveling the axles so R/L same hight and this front and behind.
Can be that bad suspension gives hanging to one side at wich also weightdifference is courced.

EDIT: looking back I see that R/L added are different from axle loads you gave , is this a miswriting or mayby by driving one side higher or lower over the scale?
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Old 11-17-2015, 12:20 PM   #13
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And just to make it a little more confusing, Alcoa rims (at least on mine) list a max of 120 psi.
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Old 11-17-2015, 12:36 PM   #14
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Quote:
Originally Posted by jadatis View Post

But do you mean that this only got less by ride height adjustment?
Correct, looking at the coach from the front on level ground it just looked tilted to my eye.

I measured a difference of 1.5 or 1.75 inches can't remember now.

I replaced the ride height rods which were fixed length only adjustable by tilting the actuator on the chassis ( really hard to do on this rig) to fully adjustable links.

Now it measures same both sides wheel well to ground, and look perfect just looking at it parked.

A bonus it handles just a bit better
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