If tire is 20% wider, should pressure be 20% less?
Thanks.

Schwab and other tire companies still put 30-35 in my tires whenever I take it in. I always take my 33 11.5 and 35 12.5 to 20-25 PSI. more even wear and more comfortable ride. BUT the more air in your tires, the better gas mileage.

If you put a wider tire on the same size (width) wheel that you had the skinnier tires on, then yeah you'll probably want to run a lower pressure to get a flatter contact surface.

Yea, I run my 12.5 tires on 8" rims. Gonna be runnnng my 13.5 tires on 8 inch rims plus weld on beadlocks. (I figure the beadlock will add about .75" on the mounting surface width.

Also have a look at the MAX psi the tire can hold and that also determines what your pressure should be...

If you want to wear down the center of your tires before the outside... I am not a fan of fast and uneven tire wear.

A tire may say- max pressure is 45 psi,okay for a F-350.But the same tire/psi on a TJ is not gonna wear well.

My buddy at the tire shop advocates starting with the pressures on your door jamb regardless of tire size. Then go from there. Always worked for me.

good advice. the psi recommended on your door is what has been tested as proper psi for the weight of the vehicle. psi will need to be adjusted for added weight from bumpers and gear. ive always liked the chalk line test. draw a line across the tread and drive a bit to check for even wear. if it wears faster in the middle then lower your pressure, if faster wear on the outside then up your pressure. i personally have 31x10.50r15s on 15x8 rims and run 30 psi.

I like the idea of starting with factory pressures and then using the chalk test to dial it in.

Hmm. Never done the chalk test. I Usually aim for an 1/8 sidewall bulge. You can also drive over concrete with water on it in one spot. Not a puddle, so the concrete has to have a slope. Drive through the wet Part and onto the dry part. This will accurately show you your contact patch.

That reminds me of a deal with fat girls. Is that where this test comes form? Something about using flour?

Hmm. What do you mean fat girls? Never heard of using flour. I just look at the track of water on the dry concrete compared to my tread width. The most important part to this is that the water isn't pooled. AlsO, start with a higher pressure and go lower 3-5 degrees between tests. Just like with the above mentioned chalk test.

Like the chalk test idea, my lift kit arrived last night so 31 tyres will be on.
seems a simple enough way to check psi.
now where is that Chalk!

I run only 20 in my 31s but they are 4 ply bias Swampers. Any higher and they wander all over the place and ride like the springs are welded together