There are some formulas also in the following document, but here's the procedure which I think is much easier:
(It will take like 5x longer to read this the first time than actually doing it, which is a 30 second affair after acquainted).
Basically, a tire's weight carrying ability at a given PSI varies based on tire size and load range. E.g. the same pressure will not support the same loads across all tire sizes and load ranges.
So the procedure is to determine your required weights, then lookup tire inflation tables to see what pressure to run for any given weight.
Here's a widely used table from Toyo tires, you can use this even if your tires are not that brand, as long as you stick to your size and load range:
To get your tire's required weight capacity, there are 2 methods:
A) Infer from the factory recommendations:
- For this you evaluate your driver door placard recommended PSI, along with how you felt about running that PSI when you were on the stock tires.
- If you liked it spot on, a few PSI higher or lower, ran different pressure front vs.. rear, etc., then take the PSI you liked and look up your original size of tire in the proper load range table. Non-Rebels and non-Max Tow trucks all come with P-metric passenger tires stock, so you'd consult pages 13-19. For example, let's say I liked running 32 psi on the stock size P275/65r18, which is a little lower than the placard instructed. So I look up 32 psi, which shows 2,502 lbs. in this table.
- Then if I upgraded to an LT tire, I'd find my new tire size somewhere on pages 20-25. (or Floatation sizes on pages 26-27).
- For example, if I upsized to an LT 275/70r18, then I'd find in the LT tables the row for that tire size, and then in that row I would find the number closest to the 2,502 lbs I got previously. (Note most sizes have 2 rows, 1 for a single wheel and another as if used in a dually... use the "Single" row).
- In this case, we see a the most similar listing of 2,470 lbs in the column for 45 psi. Since that is slightly below our 2,502 you might be fine with 45, or maybe bump up to 48 or so.
Sub-note: I upgraded to the stock Rebel tire size and load for this example. So now we can see why the placard pressures on Rebels are so much higher than non-Rebels.
B) Work from your own weights: You might do this if, for example, you are the second owner of the truck and never experienced the original tires, or if you are heavily modified and your daily driven weights are no where near stock.
- Determine the weight carrying capacity that your tire needs to meet. This isn't so straightforward. As you can see from the stock number examples above, 2,502 lbs/tire would mean the set of 4 can carry 10k lbs, well above our trucks' 7k lb GVWR. And that example's PSI was lower than what Ram recommends running at max capacity. So obviously there is a safety margin engineered into the OEM figures. Therefore, the complexity of this approach is that determining the safety margin to use is on you.
- If you can weigh your truck do so. Then you can get fancy with weight distribution front vs. rear, especially if towing or completely empty.
- Once you arrive at a weight for 1 tire, increase that by some margin to accommodate transient loads, like cornering, braking, and bumps - our previous example suggests something like a 50% margin, so there's a baseline.
- Once you have your weights, the steps are just like above steps 3-5 with the weights you came up with. Find your tire size in the tables, and locate the weight closest to your calculations, and infer the PSI from there.