18 or 20" wheel. Better for weight carrying and towing.

I ordered mine on 18s because the 20s just seem like a teenagers big fancy tires.

Just did a quick check on tirerack.com and they only have 2 tires for the Ram truck in 20" size with the cheaper Firestone TransForce (Ram factory tire) and a Michielin M/S. When looking the other day for future tires for my truck, they had a bunch of 18" available including everything from street to a true mud tire. The same Firestone TransForce (also factory) was $28 cheaper too.

Payload between the tires was the same 3640.