I know its 48 but as its on the back you need to multiply the number of engagements by the gear ratio, as you have a mod it guessing it 18-12. So that’s 18 divided by 12 giving 1.5 meaning that for every revolution of the cranks the back wheel will spin on and a half times. So the freewheel will engage 1.5 times quicker than one on the front, and 1.5 multiplied by 48 giving 72