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Old 05-07-2016, 07:10 PM   #1
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Same size tire with different circumference.

I replaced all the tires on my 2013 Rockwood 8329SS about 18 mounts ago. The original tires were junk and started to fail after only 12 months of service. I bought 4 new Power King Tow Max ST225R7515tires. The new tires are the same size at the old ones. But, the circumference of the new is larger than the old. This has resulted in the tires rubbing the underside of the trailer. I had a tire fail recently and discovered the rubbing. I replaced a failed Tow Max with a Goodyear Marathon, same size, and noticed the difference in circumference. The trailer is equipped with Lippert Torsion Axles. I mention this because they are unique and the trailer sits lower to the tire than conventional leaf spring axles. Does it make sense that tires with the same size rating would have a different diameter and circumference? Seeing the two tires side-by-side, it becomes obvious one is larger than the other.
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Old 05-07-2016, 09:13 PM   #2
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There can be some tolerance in the actual size as compared to the stated size, but it shouldn't be a large difference.

How much difference in the diameter is it?
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Old 05-08-2016, 12:27 AM   #3
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I saw the same thing with my tire size. I was very aware because I have very close spacing between the two tires on each side.
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Old 05-08-2016, 09:59 AM   #4
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Diameter should be given by a simple linear relationship:

diameter = rim size + (2 times sidewall height)

For 225/75r15 that should be 15 + ((2 x 225 x .75) / 25.4) = 28.28 inches

Goodyear has charts at http://www.goodyearrvtires.com/tire-selector.aspx

Tires increase diameter substantially if there has been an internal belt failure but I wouldn't expect all 4 to go at the same time. Maybe the Chinese mislabeled their sidewalls.
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Old 05-08-2016, 10:27 AM   #5
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Quote:
Originally Posted by DaveSchwartz View Post
Diameter should be given by a simple linear relationship:

diameter = rim size + (2 times sidewall height)

For 225/75r15 that should be 15 + ((2 x 225 x .75) / 25.4) = 28.28 inches

Goodyear has charts at Goodyear RV Tires €“ Tire Selector

Tires increase diameter substantially if there has been an internal belt failure but I wouldn't expect all 4 to go at the same time. Maybe the Chinese mislabeled their sidewalls.
Dave gave the correct formula on how to approximate tire diameter according to size, as I have also posted this before. Although there can be a slight difference in actual diameter depending on how the manufacturers measure, number rounding, and their sidewalls, it is usually fairly small.

Comparing the charts that Dave posted above for the Goodyear Marathon and this one below for the Towmax however, they both list the same as 28.3 inches.

Towmax STR Tires | Own The Road

This is odd, and sight unseen I would think perhaps the numbers may be being kicked, and you perhaps may have a 205/75R15 somewhere in the mix. Is there anyway you can take a pic of the tire label you most likely have on the side of your RV. It is most likely a yellow tag.

Also pics of the tires sizes on the tire themselves (the Towmax and the replacement Marathon) would help....before researching further.


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Old 05-08-2016, 10:35 AM   #6
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BTW: you may have just rediscovered why it is recommended that you not mix tires of different size (obvious) or even from different manufacturers on the same axle (and since RV tandem axles are so close, maybe even all 4 should be the same size and manufacturer if at all possible)
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Old 05-08-2016, 12:53 PM   #7
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Tired of tires!

Hi, had the tow max on my crusader. It lives under covered storage, but the part with the spare gets morning sun for about 2 hours in the mid morning. So, I remove the spare to the garage when not traveling. Within 2,000 miles, 18 months old according to sidewall date, (trip from the factory to Az, plus my first trip of 150 miles), all four separated. Always had to add 5-10 lbs air due to loss. Even replaced valve stems and internal Schrader valves. They all looked ok, just grew larger, and had started rubbing each other, not the frame/body. Had to replace all 4 in Payson. Not much choice in tire types up there. Took what I could get. Fortunately, they are a mid-grade tire.

Just completed a 4,000 mile trip on the new set, with no pressure loss issues, or problems of any kind. By the way, my trailer has the upgraded 16 inch, load range E tires and heavier duty optional axles. My total trailer loading is very light at 10,600 lbs as weighed fully loaded with clothes, etc. No food. (Est. add 1-200 lbs) Deduct 1,800 pin weight when hooked. (With full water, and all holding tanks 3/4-to filled, which I never travel with)

Fast forward to my next trip. Getting ready to leave. The tow max spare, never on the ground, (kept in garage when not in use) also grew in size, with the tread separating from the tire itself. Fortunately, all five tires had warranty for the tire itself. Checked pressure regularly. Always had to add air.

I hope you have better luck with your tow max than I did. Good luck, and watch them close!
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Old 05-08-2016, 01:29 PM   #8
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In 2013 I had a tire rubbing issue with my new Windjammer. Turns out the factory installed the wrong size mounting bracket for the axles which did not allow proper clearance. FR replaced the axles with ones that have the proper clearance. During this process I discovered from Dexter that they recommend at least a 3 inch clearance to allow flexing when the trailer is underway. You might check with Lippert to see what they recommend.
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Old 05-08-2016, 01:47 PM   #9
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My OEM C rated Duro tires of the same size as yours were 28.8 inches. Four years later I replaced them with Hercules E rated tires which are 28.3 inches. So in my case yes, the same size tire can be a different diameter.


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Old 05-08-2016, 01:56 PM   #10
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Yes, tires of the same size can vary greatly. We went through a spell a few years back when people wanted to replace three of the bf Goodrich tires on their trailblazers and use the matching spare for the fourth. Started having issues with the automatic four wheel drive kicking in and out on dry roads. Turns out that the OEM version was a little shorter than the replacement BFG.
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