Under-inflated: Wears the edges of the tread more than the center.
Over-inflated: Wears the center of the tread more than the edges.
Just right: The entire tread is worn at the same rate (aka, correct pressure).
If you haven't added to or removed anything from your Jeep in all the time you've owned it and are still running the stock tires in the original sizes, this story could arguably not be for you.
We don't use the tire gauge...
We don't use the tire gauge when we figure out what pressure to run our tires at. We use crayons or chalk. Draw a line across the tread of the tire and drive in a straight line until you notice wear (usually 50-100 feet). Once that's done, we note the correct pressure front and rear for reference later.
The fact of the matter is that even those "recommended tire pressures" located in your owners manual, glovebox door, or edge of the door really don't apply to you, regardless of what you might have done or haven't done to your Jeep.
OK, so why do you care if your tire is inflated correctly? Well, an improperly inflated tire will wear faster than one that has the right amount of air in it and will cost you money by forcing you to replace the tires sooner than would have been necessary with the proper pressure.
As an added bonus, if your tire is under-inflated, it will rob you of fuel economy through more rolling resistance and possibly damage the tire itself by causing it to overheat and the plies to separate. It will make your Jeep handle poorly and possibly lose the bead while driving.
A tire that is over-inflated will ride rougher than necessary, and, while you can realize better gas mileage this way, it will be at the expense of the tire's life span. Replacing tires sooner will lessen, if not negate, any fuel economy benefit you might see. Let's not forget the extra money you shell out to the dentist, thanks to all those fillings rattling loose either.
As you might expect from Jp Magazine, our "science" involves some crayons, head scratching, a couple of napkin sketches, and real-world pictures of Jeep-driving experimentation.
This is what the crayon line...
This is what the crayon line looks like if your tire is over-inflated. Notice the crayon remaining on the outer edge of the tire. Under-inflated would be the exact opposite of this, with the line worn from the edges (not from the middle).
Like Goldilocks, you want...
Like Goldilocks, you want to get the tires just right-and this is what it looks like. Never listen to a tire shop as to what pressure is right for your Jeep. Your Jeep will have different needs than every other Jeep. Figure it out for yourself. Your wallet will thank you in the long run.