I had occasion to have a chat with a FLDOT officer before I bought my dually, wondering the same thing. While they chase big trucks for other things usually, dually p/u's tend to be ticketed around here for gvwr or gcvwr. This is listed on the door, or available on the 'net. Even if you take a one ton axles/chasis and mount them under a 1/2 ton p/u, the rating on the door is what they go by unless you jump thru some hoops to change the rating on the truck, and re-title it as a one ton, and a bunch more stuff...
The R on the end of the GCVWR is just what it stands for, a RATING. So I would not consider it a S (for suggestion) but just what it is...
Make a mistake and be 500 pounds over and see just what attorney's make out of it for the other side.
David from jax