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Tires & Wheels Forum Discussion of Tires and Wheels...pretty obvious, eh?

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Old 08-07-2010, 03:15 AM   #1 (permalink)
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Load Range vs Load Index

I am considering the Nitto Terra Grapplers in 285/75R17 or 295/70R17. The 285/75's are load range E and the 295/70's are load range D. Load range E is a 10 ply rated tire and Load range D is an 8 ply rated tire. In reality the tires are likley 2 or 3 ply. The 285/75 tire has a load index of 121 and a maximum load of 3195 lbs at 80 psi. The 295/70 tire has a 123 load index and a maximum load of 3195 lbs at 65 psi pressure.

It seams that the load range E 285/75 tire has to be inflated to a higher pressure to achieve the same load carrying capacity as the load range D 295/70 tire.

Everyone tells me that I need to have a load range E tire because I tow, but I don't really see how it is any better than the load range D tire.

For comparison, the stock 265/70 tire also has a load index of 121 and a maximum load of 3195 lbs.

Can someone explain if it is really that important to to look for a Load range E tire, or is the load index the important parameter to consider when evaluating the carrying capacity and strength of a tire.

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Old 08-10-2010, 09:36 PM   #2 (permalink)
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Can anyone offer any insight???
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Old 08-10-2010, 09:52 PM   #3 (permalink)
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wish i understood the tire numbers better myself....but i have been told by more than a couple people that an E rated tire is a must for our trucks do to the weight of them.

i`m also looking at the Terra`s but the 305`s E-rated & 3640 max load.
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Old 08-10-2010, 10:11 PM   #4 (permalink)
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I ordered the Hankook atm rf10's yesterday after reading so many bad reviews on the Nitto's when on our trucks. RF10's are 285/70-17's and load range E and 126 index 3,750lbs rated. I got them for $915 installed. I'll get them Saturday. The shop that I'm buying them from said that they have seen alot of Terra's wear real bad in the middle.
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Old 08-10-2010, 10:22 PM   #5 (permalink)
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If any tire is wearing bad in the middle, it's over inflated.
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Old 08-10-2010, 10:31 PM   #6 (permalink)
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Nitto has also changed to a harder compund.... i have read nothing but good reviews about the terra`s
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Old 08-10-2010, 10:38 PM   #7 (permalink)
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The way I look at it, if they last my 35,000 or so, I'll be happy. By then I'll be ready to try something new anyhow. I go thru tires like underwear, constantly swaping, selling, or buying new rubber for one of my vehicles. But I really like how the Terra's are a good all around tire. I only bought them because they have a heavy load rating, but turns out they are quiet, handle great, and have great traction. They aren't no mud tire by any means, but they are a great all around tread design.
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Old 08-10-2010, 10:45 PM   #8 (permalink)
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Based on my personal experience with BFG AT's in various sizes and load ranges, an E rated tire normally has a stiffer sidewall for some reason. A bigger tire needs less PSI because it can hold the same volume of air at a lower pressure as a smaller tire can at a higher pressure. Think of it as the volume of the air holding the weight, not the pressure. 3195 pounds is 3195 pounds, but the D will look squishier than the E. And that's the super scientific explanation....
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Old 08-11-2010, 12:17 AM   #9 (permalink)
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Okay, here's something else I don't understand. In my example above, the E rated 285/75 tire is has a load capacity of 3195 lbs at 80 psi. The D rated 295/70 tire has a load capacity of 3195 at 65 psi.

So, who actually runs 80 psi in their tires??? If I only run 65 psi pressure in my E rated tires, could the load capacity actualy be reduced to below that of the D rated tire?

Maybe some tire guy will chime in and shed some light on this.
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Old 11-09-2010, 11:32 AM   #10 (permalink)
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The reason that the tire can run a lower pressure and have more load carrying is the larger contact patch to spread the weight. The only number that really matters is the load index. The old 10 ply vs. 8 ply is from when tires were bias constructed and not radial. They required more layers or plies to hold larger loads. This should answer you question. Towing is also a non issue because your tongue weight should not be more than 10-15% of your trailer weight and anything above 10k lbs should have a weight distributing hitch. The only thing you need to do is figure out how much you need to inflated your tires to if you change load range.
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Old 11-11-2010, 06:22 PM   #11 (permalink)
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Was at an RV semi where a retired tire engineer spoke (35+ years). He pointed out that it is the combination of pressure and volume that carries the load (Boyles Law if you remember chemistry). The 'carcass' of the tire is just there to hold the air in. Underinflating cuts the life of the tire considerably because it causes the carcass to flex more, breaking it down.
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Old 11-13-2010, 05:05 PM   #12 (permalink)
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Quote:
Originally Posted by FireBill View Post
The only number that really matters is the load index.
True, to a point. The load index is simply the max rated weight the individual tire is certified to carry without an overload-related failure. Tires fail because they get too hot and separate along the sidewall. Sidewall flex is what causes the heat, and the more sidewall flex there is, the faster it happens and the higher the temps reach.

All that being said, a tire's rated capacity is based on the volume of air it holds. A 235/85R16E at 85psi holds as much air as a 285/70 R17D at 65psi so they're both rated at 3195lbs. When you load these tires to max weight, they both do the same thing: They flex at the contact point and air pressure goes up to correspond to the reduced interior volume. Where they differ is in how much of each change they have. The D-rated tire has a much thinner sidewall, so it flexes, and most of the flex is directed outward immediately above the contact patch, causing increased movement of the sidewall, increased heat, and higher risk of failure. The E-rated tire has much thicker sidewalls which resist deformation better, thus causing the contact patch of the tire to expand lengthwise more than width-wise, resulting in less deformation of the sidewall, and reduced temperatures.

I've run both, and will never run a D-rated tire on a diesel truck again. The difference can be felt empty, and it's much more pronounced when loaded. Sidewall squirm is no fun when you're simply crossing a railroad at 65mph, but it becomes death defying when you're towing a 10,000lb trailer and have to swerve to avoid an accident.
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