80 on 305/70r16 Cooper STT Pros - what tire pressure? (1 Viewer)

[cbpeck]

This is a repost from the wheel and tire section because there doesn't seem to be much activity over there.

About a thousand miles ago I put a set of Cooper STT Pro tires on my locked '97. They are 10 ply, size 305/70r16, and were installed on freshly powder coated factory wheels. Costco aired them up to about 50 psi, and I dropped them to around 40 first, then 36, then 32. At 32 psi only about 80% of the tread was making contact with the ground (wet tires on dry pavement). I aired them down to 30 psi and now seem to be getting contact across at least 95% of the tread on pavement. They've been at 30 psi for about the last 600 miles. I've never ran tires this low on the street, but I've also never owned a set of load range E tires. I typically air down off road, so I'm not concerned about that. I just want to make sure that the on-road tire pressure is appropriate so the tires wear evenly. FYI, the Cruiser is stock, and the truck is generally empty when I use it as my daily.

I'd appreciate any advice or suggestions you guys have. Thanks!

 

[-Spike-]

Purely my opinion here, but that type of tire (I run them, in a different size and rating, 37x12.50-17, load range D) isn't designed to be run primarily on the pavement, so the typical rules don't necessarily apply. My tires are run on the street at 40psi (most of the locals run the same tires at 32-36psi, I'm an outlier there) but they see quite a bit of offroad use at 12-15psi. The edges are worn more than the middle, especially at the end of their life, because they get worn off while offroading on gravel, rocks, and such. I'm happy with the wear, it's fairly even across the 'street' contact patch. I don't think you can expect the entire tread surface to be in contact with the street with 'mud' tires.

 

[cbpeck]

Purely my opinion here, but that type of tire (I run them, in a different size and rating, 37x12.50-17, load range D) isn't designed to be run primarily on the pavement, so the typical rules don't necessarily apply. My tires are run on the street at 40psi (most of the locals run the same tires at 32-36psi, I'm an outlier there) but they see quite a bit of offroad use at 12-15psi. The edges are worn more than the middle, especially at the end of their life, because they get worn off while offroading on gravel, rocks, and such. I'm happy with the wear, it's fairly even across the 'street' contact patch. I don't think you can expect the entire tread surface to be in contact with the street with 'mud' tires.

That's insightful, thank you. I've bought quite a few sets of car tires, but these are the first truck tires I've bought. What you're saying makes sense. I'll keep it in mind.

 

[ZackR]

The larger tire won't take as much air pressure to establish a flat footprint. The OEM tire pressure is 32psi so 30psi makes sense for the larger size. I prefer a flat contact patch to maximize traction and allow for even treadwear.

I run 285/85/16's at 32psi, sometimes 34psi if I will be doing a lot of highway driving.

 

[cbpeck]

The larger tire won't take as much air pressure to establish a flat footprint. The OEM tire pressure is 32psi so 30psi makes sense for the larger size. I prefer a flat contact patch to maximize traction and allow for even treadwear.

I run 285/85/16's at 32psi, sometimes 34psi if I will be doing a lot of highway driving.

Thanks. That's the type of feedback I was hoping to receive. Just wanting to make sure that what I was experiencing made sense. I don't want to put 20k on the tires and end up with uneven treadwear.

 

[ZackR]

Search online for the "chalk test" to get a better idea. I haven't done this myself but its an easy way to verify if your tire pressure is too low or too high.

 

[cbpeck]

Search online for the "chalk test" to get a better idea. I haven't done this myself but its an easy way to verify if your tire pressure is too low or too high.

Yea, I've seen that. I think wet tires on dry pavement accomplishes the same thing. In my case, it was driving on a rainy day and pulling into a carport or garage. It gave me a clear indication that at 32 psi the contact patch was still significantly more narrow than the tire.