Ok so I hope that people running oversized tires and have off road experience chime in here, this is my first truck with tires more than one size up. Just a reminder truck is a 99 qc sb ram 2500 4x4 v10 with 35x12.51r17 grabber x3s load range E.

So the method I found was to take the max load of the tires and divide it by the max psi to find how many pounds each psi supports for that tire. Then find the weight of each axel, divide by 2 for each wheel to figure out what load each wheel is supporting then take that number and divide it by the first number calculated and then you have your psi. So I took my truck to the local CAT certified scale and got these numbers
Steer axel: 4180lbs
Drive axel: 2620lbs
Total weight 6800lbs

From generals website on working load for my size tire: 3195lbs max at 65psi max. So 3195lbs/65psi=49.15lbs/psi

Calculating front tire psi: 4180lbs/2wheels= 2090lbs/wheel. 2090lbs/49.15lbs=42.52psi in each front tire.
Calculating rear tire psi: 2620lbs/2wheels= 1310lbs/wheel. 1310lbs/49.15lbs=26.65psi in each rear tire.

So these numbers seem low to me, especially the rear tires. Anyone with experience in this area any advice would be appreciated. Right now the tires are at 54psi, tire rack shipped them with 60psi which wandered and rode terrible after I had gone through the whole front end and had it aligned.

 

Get some sidewalk chalk, mark a line across the tread of the tires. Go for a drive. See what your chalk line looks like. Worn in the center? Reduce pressure. Worn on the edges, and not the center, increase pressure. What you want to see is even wearing of the chalk line across the tires.

Another options is one of the IR thermometers from harbor freight. Go for a drive, get the tires heated up nice, then QUICKLY check one. You want even temp all the way across. Hotter in the center? Decrease pressure. Cooler in the center, increase pressure. It's a production to do it this way, as you can really only do one tire at a time..... (as they cool QUICKLY) but, do one tire on each end of the truck, and you should have a really good idea what your pressures should be.

 

I do have access to one of those thermometers. A friend suggested a paint marker but the chalk is probably quicker. I did notice the outer blocks especially by the side wall were clean on the tread face. I only have 300miles on them and don't want to wreck them, they weren't cheap.

 

I agree with HeyYou and wouldn't overthink this. Decrease pressure to the point where the contact patch is the entire tread width and not just the center portion, whatever pressure that happens to be. Chalk is a great idea, however I tend to just drive on a dusty / dirt road for a second and it is quite obvious. If you plan to tow and want to up the pressures, use the same method.

 

If you don't feel like searching for a dusty road some flour in the driveway works too

 

Well my driveway is gravel and a couple hundred feet long. Thats what first tipped me off is the center of the tread was dirty and the outer part was clean. I guess since I'm a chemist (and have a B.S. in math as well) I wanted a mathematical way then verify it by the methods mentioned above just to kinda eliminate the first guess work portion. It seems though like there is to much variance and you kinda have to it the way you have all described. I'll post back results when I have some free time to play to get the pressure dialed in. Thanks for the reaponses.

 

So my math was pretty much spot on. Verified it with a paint marker.

 

26 lbs in the rears? That seems REALLY low......

 

I'm probably going to leave them at 32psi just since i usually have a trailer on or something in the box when I use it.

 

Well 32psi is still higher than I would like according to my paint marker.