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Old 04-06-2005, 08:54 AM   #1
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Which Load Range "C" or "D"

I need to replace the Michellin 7 x 15 load range C tires on my 79 Ambassador (4650#dry, 2 axle). I am planning on Marathon 225/15R75's. The Airstream owners manual recommends 45psi for radials. Should I use load range C (50psi max) or load range D (65psi max)? Is there any issue in using a "D" tire at 20 psi below its max pressure? Or, will either work just fine?
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Old 04-06-2005, 02:40 PM   #2
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Yokohama still makes 7.00x15 tires, and we replaced our's last summer when we purchased our trailer.

It is critical that you take this advice and include it with all other advice you get...

Running any tire lower than the optimal pressure will reduce the tread life of the tire. It will also impair the optimal design of the tread contact with the road. Altering this surface contact with less than ideal inflation will alter the performance of the tire in driving and breaking conditions. Running "soft" will also apply additional wear to the sidewall of the tire, and increase the risk of a blow out over time.

In My Humble Opinion, you should consider replacing with the Yokohama tire in the Load C, or go with the Load C radial.
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Old 04-06-2005, 04:53 PM   #3
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I just replaced all 6 of mine. The original tires were a C range Goodyear Marathon, there were some problems with that particular tire a few years back. Goodyear recommended going with a D range tire. C is rated at 2050 lbs D is rated at 2540. I went with the D range just for the weight saftey factor. They will be inflated to max pressure for saftey, fuel and tire economy.
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Old 04-06-2005, 04:57 PM   #4
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Marathons

Hi. Put a set of Marathons (Ds, single axle) on my Globetrotter a few years back. I was quite surprised in the difference between the bias belted and the radials. Significant improvment.
I recall a few months back someone posted a link to a table of PSI in relation to the loaded weight on a tire but couldn't find it just now. Made a good case for weighing the individual tires for load. Too much air can be just as bad as not enough.
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Old 04-06-2005, 05:21 PM   #5
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Quote:
Originally Posted by sovereignrwe
Running "soft" will also apply additional wear to the sidewall of the tire, and increase the risk of a blow out over time.

In My Humble Opinion, you should consider replacing with the Yokohama tire in the Load C, or go with the Load C radial.
Goodyear says that 15 to 20% underinflation causes excess sidewall flex and casing damage. I would stay with the load range C.

Putting 45 psi in a tire rated for 65 psi seems too soft for me.

When you get your new tires, weigh the trailer in it's fully loaded condition to determine what your real inflation pressure should be.

I'm putting D's on mine, but it is a single axle, like Tom's.
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Old 04-08-2005, 08:08 AM   #6
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Post Thanks for the advice

Thanks to everyone who responded both in the thread and via PM.
The consensus is to stick with Load range C, so I'll have my local tire place order a set.
Our first trip of the season is coming up soon. Can't wait to get the silver palace back on the road!
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Old 04-08-2005, 10:27 PM   #7
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Quote:
Originally Posted by markdoane
Goodyear says that 15 to 20% underinflation causes excess sidewall flex and casing damage. I would stay with the load range C.

Putting 45 psi in a tire rated for 65 psi seems too soft for me.

When you get your new tires, weigh the trailer in it's fully loaded condition to determine what your real inflation pressure should be.

I'm putting D's on mine, but it is a single axle, like Tom's.
<--- son of an independent Goodyear dealer (31 years independent, several more with Goodyear, before going independent.) : )

I've also changed a tire or two, but several hundred thousand short of my father.

I have stayed away from the D's as I have dual axles and can see where the extra stiffness in the sidewall can increase the beating the trailer takes. I won't suggest that I know everything there is to know, nor that anyone is wrong in putting D load tires on a trailer. I can only offer my opinion, which is free and worth every penny.
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Old 04-08-2005, 10:38 PM   #8
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Quote:
Originally Posted by Tom Nugler
Too much air can be just as bad as not enough.
Tom.
So very true, Tom. Over inflation, while not as hard on the sidewalls, will cause the wear in the center of the tread to be greater than the edges. Just as with underinflation, it will change the area of tread that is in contact with the road surface and impact cornering and stopping.

On a car or truck, both overinflation and underinflation will impact the life of a tire, as tread wear tends to be the greatest factor in the life span of these tires.

RV and trailer tires tend to be used differently. They generally don't see the same number of miles a year, and thus do not consume the tread of the tire before the tire deteriorates due to natural conditions. This is one of the reasons why we buy tire covers for RVs and trailers and not for our cars. I am of the opinion that underinflation is a greater hazard on lower annual mile tires.

I do agree with your statement, I just thought I would pull a Cliff Claven and too much information in an effort to explain your position.
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Old 04-09-2005, 07:31 PM   #9
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Any of the tire manufactures has load/inflation charts on their web sites for download. The load/inflation for an ST225-75R15 is 1260/20, 1430/25, 1600/30, 1760/35, 1880/40, 2020/45, 2150/50(C),2270/55,2380/60, 2450/65(D)
From Goodyear's web site "Correct tire inflation is a key component in tire care. The recommended maximum inflation pressures for your tires are indicated on the certification label or in your owner's manual. Since RVs can be loaded with many different configurations, the load on each tire will vary. For this reason, actual air pressure required should be determined based on the load on each individual tire. Inflation pressure should be adjusted to handle the tire carrying the heaviest load, and all tires on the axle should be adjusted to this standard." Note that Goodyear says that inflation should be based on load on the tire, not maxium permitted.

Based on your trailer weight,there is no reason to go to a "D" tire. You should get good weights with the trailer fully load and set tire pressures accordingly, with some extra pressure for safety. No need to go to the full maximum pressure, just enough to carry the load. Added pressure just beats the trailer apart, reduces contact patch and traction.

My 2002 Safari SS weights 6500# fully loaded and carries 6100-6200 on the axle when hooked up with Reese equalizing. Based on this, 30 psi should be sufficient. I run 40 psi and they run nice and cool. This gives me about over 10 psi safety factor. Even if they lose a few pounds, I still have plenty of weight capacity.
Everything in the trailer stays in place, even on the rough Michigan roads. I see no reason to beat the trailer apart by running 65 psi. With that much extra pressure, I will be reducing the footprint and traction. Not the thing I want to do.
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Old 04-18-2005, 07:47 AM   #10
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Post New Tires

Well, I picked up the new tires last week, about $98 ea incl. balancing and taxes. Marathon 225/75 R15 loadrange C. I swapped them on and off the trailer myself after previous experiences with "suggested jacking points" in the owner's manual (I pushed a hole thru the belly pan at the "jack here" label). The only downside is the cast aluminum tire clamps no longer work. The diameter of the Marathons is smaller than the old Michelins and there is not enough adjustment to hold them, so it's chock time.

As suggested in another thread, I repacked the bearings yesterday. With gathering tools and cleanup, it was 3 hours, but it was nice to see the bearings were all in good shape, the brake pads were fine, the magnets had little wear, and the PO had left me a stash of a dozen grease seals.
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Old 04-18-2005, 07:54 AM   #11
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[QUOTE=sander17](I pushed a hole thru the belly pan at the "jack here" label)./QUOTE]


What they don't tell you is you are supposed to place the jack slightly inboard of the "Jack Here" label. If you are able to get under the trailer enough to see, there is a line of rivets in a line from front to back just inboard of those labels, that is the frame, and you need to place the jack under that, in line with the rivets. A small aluminum patch, vulkemed and riveted in place over the tear, will keep dirt, rodents, and other uninvited guests out of that hole.
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Old 04-18-2005, 09:11 AM   #12
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Quote:
Originally Posted by Tarheel
I just replaced all 6 of mine. The original tires were a C range Goodyear Marathon, there were some problems with that particular tire a few years back. Goodyear recommended going with a D range tire. C is rated at 2050 lbs D is rated at 2540. I went with the D range just for the weight saftey factor. They will be inflated to max pressure for saftey, fuel and tire economy.
Hi Rick! May I ask where you got your tires. We're trying to get the old beast back on the road. I know you're just a tad east of us, but it'll give me a backup plan.

THX!
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Old 04-25-2005, 07:26 PM   #13
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Required amount of tire pressure in trailers

I am a little unsure about the amount of air I should have in the tires on my travel trailer. I have a 2004, International CCD. On the plaque it has a gross weight of 7300, the size of the tires and the air press should be 65. However, the Good Year manual gives different amounts of air for different weights. Any idea on what the air pressure should be?

Thanks!!
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Old 04-25-2005, 09:50 PM   #14
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I have a 96 30' Classic, the tires at the time from the factory were 'C' range, now they put 'D' range tires on the 30' Classics. I know the newer trailers are a bit heavier but not that much heavier. I replaced the 'C' with 'D' range Marathons and I'm glad I did, safety factor... One day I put a half can of coke on the counter by the sink when I was working on the trailer, I forgot the open coke can was there and I then took the trailer on city streets and the highway back to the storage area 15 miles away. I opened the door to the trailer and the coke can was still on the counter where I left it. I don't think my trailer has a harsh ride with the 'D' range tires.
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