People often ask me how much air they should run in their tires on the street. It's nearly impossible to answer a question like this because of different tire constructions, load ratings, vehicle weight, and so on. But typically these people want the best MPG they can muster, their tires to last as long as possible, and they don't want to sacrifice on-road comfort and traction. There is a simple solution. All you need is a piece of chalk and a smooth and level road or parking lot.
Run some chalk across the tread in a solid line. Then drive with the steering wheel straight over the smooth flat street or parking lot for about 50 feet. Stop and see where the chalk is worn. If the chalk is smeared off only in the middle you have too much air in the tires. If only the outer edges are smeared off and the center appears mostly untouched, you have too little air. You want the chalk to wear evenly across the entire tread. Adjust the air pressure front and rear until you achieve this.
The chalk trick works on most light-duty vehicles. But it is especially helpful on lifted 4x4s with aftermarket tires that often have an increased load carrying capacity over the stock tires.