Ignoring changing duty cycles result in equipment mismatch which increases operating cost and misallocation of resources. The objective of this study is to investigate the effect of changing duty cycles on the optimal shuttle car fleet size used to mine a particular panel width. A room and pillar mining system is modelled using discrete event simulation and validated using data from a real mine. Experimental analysis is conducted to evaluate the effect of ignoring changing duty cycle in a 17-entry coal panel. Changing duty cycle is accounted for by determining the optimal fleet size in defined segments within the panel. The study shows that, for the 17-entry panel studied, a fleet of four shuttle cars is optimal for the entire panel when changing duty cycle is not accounted. However, by accounting for changing duty cycle, a fleet size of four is only optimal for 80% of the defined segments.