Tire pressure for “E” rated tires

[Bdog]

I have Cooper STT Pro 35x12.50x22’s on my factory wheels on my 1500 longhorn crew cab. My truck never hauls more than me, the wife, and two kids and maybe a couple hundred pounds of misc junk. I have a 3/4 ton diesel for real truck things.

The tires say 65 psi max on them for a load rating of 2835 per tire which equals 11,340 lbs for the four. I am guessing the truck weighs maybe 6,000 with us in it?

I have about 9k miles on the truck (and tires) and have been running them at 55 psi and it rides rough. I can really feel the bumps. I dropped them to 45 psi this morning and it was a noticeable improvement but still no Cadillac ride. It seems in theory the lower the tires go the smoother the ride as they will flex and absorb a lot of the bumps before they even get to the suspension.

How low can I safely go? 40 psi? 35 psi?

I have a semi and for the tires on it there are charts on psi for certain loads. Like the max load is at 120 psi, 115 is good for x, 110 is good for y, 105 is good for z and so on. I can’t find any such chart for my coopers. Any ideas?

 

[Rob5589]

Low profile tire and E load aren't going to be soft riding for sure. Personally, I would run them as low as possible that isn't showing abnormal/underinflated wear patterns.

 

[Bdog]

Thanks. That is the idea but how long do I have to go to notice an abnormal wear pattern? I figured this would be a fairly common issue. Is 35 psi too low to try? I guess my fear is going to low and losing a tire on the freeway.

 

[Rob5589]

You could use the old school method and run a chalk or tire crayon line across the tread and see where it gets worn. Or after a couple of hundred miles you'll be able to see what the tread looks like.

 

[Awd1990tsi]

Thanks. That is the idea but how long do I have to go to notice an abnormal wear pattern? I figured this would be a fairly common issue. Is 35 psi too low to try? I guess my fear is going to low and losing a tire on the freeway.

I’ll tell ya in my experience of towing with 35x12.50r20 E range MT tires I had them set at 44psi for around 8-10 thousand miles (50% loaded, 50% not) and was seeing more wear in the centers than the outside of all 4 tires. And it rode pretty bad. Dropped them all to 36 psi like the door sticker called for on factory tires and the ride is a lot better. Took it on a couple runs since, so ~2,500 miles and haven’t seen the center wear appear to get worse. Measuring tread depth the center is still about 1/32” less than the outside, but it was closer to 2/32” before I dropped pressure.

If I keep towing with it as much as I have been I might bump the rear tires back up to 38 or maybe 40psi. But only if I do start to see edge wear surpassing center wear.

 

[DavidNJ]

Attached PDF covers how to calculate equivalent pressures