There is no IF. According to Tire Rack tire specs, the tires on the 20s are 28.7 inches tall (diameter) and come in at 729 revolutions per mile. The tires on the 18s are 28.4 inches tall and come in at 731 revlutions per mile. Not enough to cause a speedo calibration issue. You could nearly get that spread of actual tire height and revolutions per mile difference on the same tires just with poor tire pressure managment or differences between new and worn tires.

The difference in gears is purely for performance (real or presumed), not to compensate for tire diameter.

You could feasably order the Challenger with 20's and the taller gears (numerically lower) from the dealer and there would be no difference in speedo calibration. As another example, the automatics do not get different gears with the 20's vs. the 18's.