Excerpt from: http://www.fourwheeler.com/techartic...y/viewall.html
Bigger Tires, New Pressure
Once you install oversized tires, the OE-recommended pressures on the tire placard, or in the manual, are no longer valid because the internal volume of the tire has changed. Here's how you can translate those OE tire pressures to a new tire and give yourself a starting point. Increased volume usually requires less pressure to carry the same weight in the same class of tire because it isn't air pressure holding the tire up, it's air volume.
First, you need to find the Tire and Rim Association load inflation table (LIT) for the size and type of tire you are switching from, and to. If you Google "Load Inflation Tables," you will find many sources of such info. The tire manufacturers will also supply this information directly, and tire shops often have it.
: Look at the tire placard on your vehicle (or owner's manual) and write down the pressures listed for the stock tires. In our case, they were 50 psi front, 60 psi rear for LT245/70-17 tires, which will be our example. Those are the pressures listed for the truck to carry a rated capacity load, but yours lists both loaded and unloaded specs; either will translate.
Look up the old tire on the load inflation table and note the weight each tire is rated to carry at the psi rating you want to translate, in our case 50 and 60 psi.
The LIT is broken up into 5psi increments, and your recommended pressure may fall between. Yes, it matters. If that happens, you will want to determine how many pounds of weight that 1 psi will carry. Let's say your pressure is 47 psi and you are going to do the same tire swap we are. The weight rating at 45 psi is 2,010 pounds, and at 50 psi it's 2,205 pounds. Use the following formula:
2205-2010 lb.= 195 lb. = 39 lb. per psi50 psi-45 psi 5 psi
Subtract 45 from 47 and you have 2 psi. Multiply 39 x 2 = 78. Add 78 pounds (2 psi worth of weight) to the 45psi load rating and you get 2,088 (2,010 + 78 = 2,088). That's the load the tire can take at 47 psi.
Look up the new tire on the LIT. Find the pressure needed to hold up the same weight at the rated placard pressure. Our new LT285/70-17 tires can carry 2,105 pounds at 35 psi, so using the formula in Step 3, we know that to support 2,205 pounds, we need 38 psi. In back, our 60 psi load rating dropped to 44 psi with the larger tires.
Using the attached Load Inflation table I calculated that I should run 40 PSI with my 275 70 17s. I have been running 31.
Try it and see what psi you get.