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Old 08-27-2013, 10:00 PM   #29
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1979 31' Sovereign
Northeastern , Kentucky
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I appreciate the rundown on derating tires, and specifically on the 235 75 R15 (P) Michelins. Very helpful and it appears these tires are more than adequate for the larger (31 foot) 1970s model Airstreams (I've got a 79). Any issues with the stock rims and these tires?
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Old 08-28-2013, 08:53 AM   #30
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2013 25' FB International
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For Michelin part number 36210 which is the P235/75R15/XL 108 T tire:

UTQG Treadwear, UTQG Traction, UTQG Temp is 720 A A
Rim width range is 6" to 8"
Section Width On Measuring Rim Width is 9.3" on a 6.5" rim
Diameter is 28.9"
Tread depth is 12/32
Derated load is 1,985 pounds at 50 psi
Tire weight is 31.9 pounds

Read all about it:

LTX M/S2 | Michelin Tires

I installed this tire on the stock model year 2013 15" rims (both the aluminum and steel spare wheels) for a 25FB International. Older wheels may have a different rim lip design, so check with a tire professional to be sure the older wheels are compatible with these newer radial tires.
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Old 08-28-2013, 11:16 AM   #31
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Quote:
Originally Posted by switz View Post
Here is the reason for derating the Michelin LTX (P) 235/75R15 XL sidewall rating of 2,183 pounds @ 50 psi to 1,985 pounds:

*************************************
49 CFR 571.110

Tire selection and rims and motor home/recreation vehicle trailer load carrying capacity information for motor vehicles with a GVWR of 4,536 kilograms (10,000 pounds) or less.

S4.2.2.1
Except as provided in S4.2.2.2, the sum of the maximum load ratings of the tires fitted to an axle shall not be less than the GAWR of the axle system as specified on the vehicle's certification label required by 49 CFR part 567. If the certification label shows more than one GAWR for the axle system, the sum shall be not less than the GAWR corresponding to the size designation of the tires fitted to the axle.

S4.2.2.2
When passenger car tires are installed on an MPV, truck, bus, or trailer, each tire's load rating is reduced by dividing it by 1.10 before determining, under S4.2.2.1, the sum of the maximum load ratings of the tires fitted to an axle.

S4.2.2.3
(a) For vehicles, except trailers with no designated seating positions, equipped with passenger car tires, the vehicle normal load on the tire shall be no greater than 94 percent of the derated load rating at the vehicle manufacturer's recommended cold inflation pressure for that tire.
(b) For vehicles, except trailers with no designated seating positions, equipped with LT tires, the vehicle normal load on the tire shall be no greater than 94 percent of the load rating at the vehicle manufacturer's recommended cold inflation pressure for that tire.
********************************

As mentioned above in a previous post, LT tires are not derated.

Thus the bunkhouse trailer with a GVW of about 8,800 pounds exceeds the capacity of the 15" Michelins (4 x 1985 = 7,940 pounds).
Thanks switz. I said LT thinking of the LTX...you posted the DOT standard I referred to. Thanks.
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Old 11-12-2013, 11:30 AM   #32
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Wouldn't another consideration for this topic be the weight of the current steel wheels? For me that is going to be a more important consideration. When these new 15" tires need replacing I'm hoping that I can afford to go to aluminum wheels that will reduce weight. At that point I'll wrap them in the appropriately sized tires. If I can pick up inexpensive 15" alum wheels I'll probably stay at that size since my trailer has been rolling since 1973 on the 15's it came with.
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