I think it's more the tires than the rims. Typically, the 20" tires aren't setup for as high psi as what are available for the smaller rims and I've always heard you should run the psi the truck manufacturer recommends per tire, not what the tire company says the psi should be. So, even if you have non factory tires with a heavier tow rating or max psi, I'm not sure it will help much. My owners manual doesn't mention this fact anywhere but I did call Dodge customer support and they confirmed it as a fact.

 

Trailer Life, tow ratings. Any year, any make. Believe me, it's detailed and very accurate.

 

I run the pressure that's best for my tires. The placard on the vehicle is for the stock tires for a stock ride. The manufacturer has no idea what tires you're going to choose in the future.

Remember the whole Explorer fiasco? All that was caused by Ford's inflation pressure recommendation being too low. The tires would have been fine if they weren't set for "marshmallow".

FWIW, I run 38 psi -- 42 in the rear when towing -- and the tire's max is 50.