Originally Posted by kowbra
Good info!
But, aside from all of the regulations, and to all who ask "who cares?" - I like that you have properly described this as a "safety violation". Going way up in tire size and capacity can actually make one less safe, not more safe. And, that is why the regulations exist. But that is not the whole story...
I will add to the load range discussion... it is not as simple as just comparing the Load Ranges, like "D" (LRD) or "G" (LRG), etc. In fact there are lots of load ratings within each broad category. These are the 3 digit numbers often called "load indexes" that have a corresponding weight rating. For example, a load index of 125 will always have a capacity of 3640 lbs, regardless of whether the tire has a load range of "D" or an "E" or "F", etc. You are correct in saying that if a tire has the same load index number, it will use the same tire pressure and the carrying capacity will be the same.
BUT, there can be a lot of variance within load ranges. For example, some LRE tires could have a load index as low as 119 (3000lbs). Yet some LRD tires could have a load index as high as 124 (3525lbs). IOW, in the above examples, it is possible to have a "D" tire with more carrying capacity than an "E" tire. (similar with "F" and "G" ranges, although they tend to have a narrower spread)
So, where load ranges like "E" or "G" mostly refer to the number of plys of construction (more related to stiffness), the 3 digit load index narrows it down to what carrying capacity the tire actually is rated for.
In the case of the OEM tires that the OP discussed, I "think" they are load index of 124 (3525lbs) but I don't know for sure what brand came with his rig.
So to go to the Sailun "80's" is likely an upgrade, as the Sailun G637 in size ST235/80R16 has a load index of 129 or 4080lbs. The "85's" have a load index of 132 or 4400lbs.
Regulations aside, where might a practical safety issue come in? If the original rim has a load index much lower than the new tire, you could create a safety issue due to differences in max inflation. The real key is that with the G rated tires, you will be running a tire that needs 110psi in order to achieve its load index. It is critical to make sure that your RIMS are rated for 110psi, or you could definitely cause a safety issue. Rims rated for 85psi can and have failed if tires inflated to 110psi are installed.
So, to the OP, if you are using your original rims, I think it is very important to make sure that your rims are rated for 110psi, and ideally have a load index of 129 or higher. Look at the back of the rims to confirm. Your rims may be fine, but if not, make sure to buy new rims with those new tires.
As to whether to go to the G rated tires in the 80's profile vs the 85's? It's your choice, but the 85's will have no practical increase in carrying capacity, because both greatly exceed the axle rating. And both would technically be a safety violation. The only practical difference is that the 85's have a bit more tread depth to start with, so could potentially last a bit longer. But, for the weight they are hauling, the longevity will likely depend more on proper inflation than the difference in tread.
hth!
Brad
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