How to determine tire pressures; 255/75-17 C
 

I am going from a 265/70 to a 255/75 on my 5th gen. Just wondering what tire pressure I need to run and what is the lowest I should air down to for sand/beach? I think I run 38 cold and air down to 15 on the beach.

Obviously I can't go by what the manufacture recommends. Vehicle is 4750lbs with a GVWR of 6300lbs on a 7" wheel.

More info,
255/75 is the OEM tire on JK Rubicons. It has a 7.5" wheel and weight 4521 and a GVWR 5700. Google says OEM recommended is 37psi.

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Thanks

This requires time and patience but the psi numbers with this method work well.

Do this on a relatively flat surface without any cracks or bumps. Inflate the tires higher than what you normally keep the psi at without going over the max inflation number on the wall of the tire. You will have to do two sets of tests for the the two psi numbers. One for DD and another chalk test with the truck loaded up.

Draw a thick, straight line across the width of the tire. You’ll want to “chalk” the tread blocks all the way across, in a straight line including the outer lugs. You may need to chalk two rows to get a good covering.

Now drive your vehicle in a straight line forward for about one complete car length.

Get out and inspect the chalk mark left on the ground and on the tire. A tire with the proper air pressure should press the chalk line evenly across the ground. This means you'll see the entire chalk line imprinted on the ground. An over-inflated tire will bulge (or crown) and only the center of the line will touch and be left on the ground. In this case you will only see a small portion of line left on the ground. Since you are aired to higher inflation, then this is what you should see. Now drop the psi in tires by about 3 psi increments until you get the the correct number.