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Old 06-30-2010, 08:48 PM   #1
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Tires, Axles, and GVWR

Okay, so, I look at one of the tires on the Rockwood 8318 (2008) and notice that I have a pretty bad tire (it appears to be dry rot, but worse).

Anyway, I look at the Carlisle tire - ST205/75R15 C. Stated capacity of 1820 lbs @ 50psi.

This makes me think... 4*1820 = 7280 lbs of capacity. Hmmm... Sticker on the camper (I bought camper new in 2008) says GVWR of 7700 lbs. I do know that the tongue weight of the camper is around 700lbs.

So, then I go to the local trailer place to discuss some tires - I figure time to upgrade to some 225/75R15 D range tires to give me some added capacity -- I noticed a little cupping on both sides of all 4 tires. The tire guy (whom I trust) says these are 5-lug tires -- 3500lbs per axle. So, quick math in my head... 7000 lbs. Again, this is REALLY REALLY not good! Yeah, I know, 700 lbs on the hitch, but really, come on, do the MFR's really cut it this close?

Has anyone else noticed this extremely tight tolerance on the Tires & Axles?

Would it really be a waste of my money to upsize to the 225's?

Just really confused... I do have about 15K miles on the current set of tires and they actually - basically look pretty decent. But, I think it is time to change them all out.


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Old 06-30-2010, 10:46 PM   #2
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Location: Fort Collins, CO
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I think it adds up. 1820 lb x 4 tires + 420lb tongue weight = 7,700. You are correct though, there is no margin. When the maximum weight rating says 7,700, it is really 7,700 pounds. I don't know if the tire and axle manufactures have designed in any margin, but the rated capacity is undoubtedly factual, accurate, and to the pound.

You shouldn't have to move up to the wider 225 to get a load-rated D tire. D-rated tires are available in your original size. It's your money. If you want a D-rated tire, then go for it. (I have contemplated going to the D-rated tires too.) BUT I wouldn't equate that to increasing the weight rating of your TT. I would still assume 7,700 lbs. All you are getting with the D tires is a 2 more plys and a 65 psi tire pressure.

If you by the D-rated tires, and fill them to 50 psi, you still only have a weight rating of 1,820 lbs per tire. If you put 65 psi in the same tire, you now get 2,150 lbs per tire. BUT, again, we don't get to print up a new label for our trailer and increase the GVWR just because we put on higher capacity tires.

Also, be careful about mounting 225 tires where 205 is specified. You may find that the new tires will rub against the body of the trailer when they bottom out or the trailer is loaded. That is a good way to wear out a tire ahead of schedule.

Jim P.
2008 Surveyor SV-291
2003 Ford 4x4 Expedition
Reese SC WD System 800/10,000 lbs
1976 Cessna 177RG
Wife, son, twin daughters, and mother

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Old 07-01-2010, 06:15 AM   #3
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Yep, I know that the extra tires technically doesn't increase my carrying capacity, I think it was more for peace of mind on those really long runs where the speed can get up there. Start getting in the mountains when the road temps are pushing 100, get rolling/coasting down a grade and speed can get up (70-75) pretty quick - not that I like it. Just figured i'd rather have some extra "margin" on things I can control.

Yes, I do "try" and keep my speeds to 65, but sometimes the passing trucks, hills, and whatever inch the speed up and before I know it, i'm sitting on 70. With the Expedition that I used to pull with that wasn't an issue, but with the new F250, it can happen (and does) real easy.
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