Life and death is the importance of having the right load rating for your load. For starters you want at least the load level of tire that came factory and no less. Higher load rating is fine, but less is not. Here is a site you can get some help with your query. http://www.tirerack.com/tires/tiretech/ ... 25051516:s
In reality, the number of plies doesn't mean as much as it used to. What Rambodog said, it's all about the load range. With the steel body ply and belt design, I believe most radials are 5 ply. The higher load range tires have beefed up steel cables inside the carcass. It's best to determine what your maximum load will be and not choose a range that's lower than that number and keep an eye on your pressures. Rambodog said it all.
One thing to add. Providing your tires are load rated per OE spec, you can tug 6800 lbs. on your 3.08's. This is about 85% of your capacity. This is a decent place to be in terms of safety and truck reliability. Increasing load capacity of tires is fine for a bit more sure footing in turns as the sidewalls have less give. This in turn can make for a stiffer noisier ride the higher the tire load capacity when truck is driven unloaded. Airing down a load E tire to obtain a C load or P tire quality ride doesn't work either. The tire can distort and cause vibrations similar to a tire out of balance at highway speeds. Watching a Top Fuel cars rear tires going down a track in slow motion is a real exaggerated example of what a tire can do when spinning fast. Granted they are wrinkle wall and way exaggerated it is distortion non the less on a larger scale. Our tires get taller and skinnier at 60 mph. I never would have thought any real amount to mention. I have a video of me doing a burn out at 50 or 60 mph and it's subtle and quick, but quite noticeable that it happens.
Start at :50 and see how distorted it is.
Airing them at door jamb psi. isn't enough. I learned the hard way seeking comfort. Well, too much comfort we will say. Minimum of 45 to 50 psi for me. My Load E's call for 80 psi. max load cold. I run them currently at 48 in front and 44 in rear. Lighter in rear of truck hence the lower psi to avoid rear bump hop when running empty or unloaded. Any softer and problems arise and much harder it takes from the comfort. You will have to find what works for you loaded and unloaded once you get the load rating issue resolved regardless of tire. Keep this in mind if tires are to be replaced, especially with a high load rated tire.
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