Ram calculates the load range for each axle and wheel for the truck, including extra forces from hitting potholes or offroading over rocks. They then use the tire manufacturer's load/inflation tables to determine what pressure to inflate the tires to, in this case about 2900 lbs load, which Toyo's tables point to 55psi for the front tires.
Ram recommends lower pressure on the rear tires, at 45psi, unless the truck has a big load or is towing. Then they recommend 55psi. You might get a small % better mileage if you're lucky, but you will get a slightly rougher ride and reduced traction in the rear because less tire is resting on the pavement. I don't like running higher than recommendations because of the traction issue. Especially on wet pavement.
There are some really long discussions from last fall and this spring, in one of the subforums, that are worth reading.
Also, there are lots of guys that will argue for higher or lower pressures. Most dealers are not engineers. They aren't working off the same sheet of music that Ram and Toyo are using.
But the tire manufacturers across the board make tires to varying specs, and they generate the same kind of load tables. And they ALL recommend that you follow the vehicle manufacturer's guidelines for proper inflation, if you are using the stock tire. If you switch to different tires, just use the inflation tables for the new tires.