calculating sprocket RPM is very easy if you know what the RPM of the drive sprocket is. If you know that then it is a simple fraction problem. If you make the drive gear the numerator or the top part of the fraction and the driven gear the denominator or the bottom part of the fraction (DRIVE/DRIVEN) I.E. if the drive sprocket has 25 teeth and your driven sprocket has 100 teeth (25/100) that will reduce to (1/4) or a 4:1 ratio. So if your drive sprocket is spinning at 1000 RPM then your driven sprocket will be spinning at 250 RPM. You can plug your specific sprocket tooth count into that equation and come up with the right answer.