-The overall width of both inner and outer wheels bolted together.

-Distance from hub to inner lip

-Distance from hub to outer lip

-Distance between both rear wheels

-Tire bead seat width

The DOT minimum space between the rear dually tires is 1/2″ (1.27cm). safe at maximum load capacity and will 

prevent small debris from getting lodged between the rear tires which could result in premature wear and punctures.

Depending on the tire size - spacers between the duals, or

 on the hub could be necessary to give more clearance.