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Old 06-13-2018, 07:04 PM   #1
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Putting a 315/80/22.5 on a 8.25" rim instead of 9"

Both Firestone/Bridgestone and Continental approve using their 315's on 8.25" rim with the following footnote:
"8.25-rim may be used if the tire load is limited to 8,000lb. single and 7610 lb. dual @ 120 psi." This is 3% less than the 8270 lb that a single tire is capable of at 120 psi so some do long dissertations that you have to decrease all numbers in the load table by 3% negating much of the reason for going with the 315 over a 295/80/22.5. (7580 @ 120) a 690 lb difference.

BUT, the important part is "7610 lb dual @ 120 psi" which is NOT a reduction in load carrying on a 8.25 rim. The reason for limiting a single at 8,000 lbs is the weight rating of the rim, not because it's 8.25" wide. All 8 of my rims are rated 7650 lbs @ 125 psi...fortunately I do not need 7650, much less 8,000 lbs.

My front axle is at 14,000 lbs with almost perfect balance. A 295/80/22.5 requires 109 psi but a 315 requires 96 psi...a 13 psi difference. 13 less psi definitely makes for a smoother ride. I did not buy into having to derate because I put them on a 8.25" rim and having put 40K miles on Firestone FS 400 315's on the front of my 04 Dynasty, I was a little surprised that they had slightly more edge wear on all 4 edges of the tires at 40K miles.

I'm replacing the 4 year old 295 CRACKING Michelin's on the front of my new ride with Continental 315 HSR2-SA. The FMCA price is only $30 each less than Atlanta Commercial Tires price, BUT they are out of stock. Should mean they will be made in 2018 when they get them in next week.
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Old 06-14-2018, 07:24 AM   #2
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Double post.
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Old 06-14-2018, 08:36 AM   #3
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Quote:
Originally Posted by Ivylog View Post
A 295/80/22.5 requires 109 psi but a 315 requires 96 psi...a 13 psi difference. 13 less psi definitely makes for a smoother ride.
Will the decrease in pressure make for a softer ride (with a larger tire)? Are you assuming this - or do you know this?

I wonder if a lower air pressure acting on a larger surface area could exceed the force exerted by higher air pressure on a smaller surface. I don't know.

96 psi x 1000 sq-in = 96,000 pounds force. Hard ride?
109 psi x 800 sq-in = 87,200 pound force. Softer ride?

I don't know the actual surface area of the tires - I'm just using 1000 and 800 for the math example. But it would be correct that the 315 tire has a greater surface area than the 295 tire.

Just curious...
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Old 06-14-2018, 11:41 AM   #4
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My 295/80/R22.5 Michelins are rated at 7830# @120 psi, not 7580#.
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Old 06-14-2018, 11:44 AM   #5
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Our previous coach had 315 front tires and I think they provided a slightly smoother ride.
We purchased new 315/80R-22.5 front tires for our current coach last weekend. It previously had 295/80R-22.5 front tires. I dropped the pressure from 120 to 110psi.
There is a noticable ride improvement and less tramlining.

I tried to purchase 9" wide wheels to replace our current Accuride #29540 (8.25" wide wheels). I spoke to Brandon at Accuride and he says because the wheels are made to be interchangable front to back, that the 9" wheel has a different offset than the 8.25" to accomodate the required dual spacing. Also, the Monaco "D" shaped holes are no longer available.
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Old 06-14-2018, 11:51 AM   #6
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Quote:
Originally Posted by Crasher View Post
My 295/80/R22.5 Michelins are rated at 7830# @120 psi, not 7580#.
Yes, Michelin can carry the same weight with 5 less psi...must be their stiffer sidewalls as all other manufactures load tables are 7830 @ 125.

"Will the decrease in pressure make for a softer ride (with a larger tire)? Are you assuming this - or do you know this? My butt knows this as I used 295's for 40K miles on my Dynasty before going to 315s. It's also Pounds/ Square Inch...less pressure equals more cushioning...a softer ride regardless of the area.

Some Monaco's on the RR10S chassis do not have enough room to put a 315 on the front without hitting the RF airbag mounting plate. It was a close fit on my 04 Dynasty but on my 08 Navigator there is plenty of room , even with a wider door.
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