JRW is spot on. Why put yourself through all that pain when you can just go and measure it?
Even if you don't have a tacho, just put two texta marks on the conveyor a known distance apart, and another one on the belt. Time the mark on the belt between the marks on the conveyors, repeat 5-10x and do an average of all your results, then calculate the speed from there.
It's quicker than crawling around trying to dust off gearbox nameplates and do complex calculations, and allows for Bubba having put the wrong nameplate on the motor at the factory (which would never happen, of course...)