How is the drive ratio of a roller chain drive calculated and what is the maximum recommended single reduction?

The drive ratio is a function of the number of teeth on the large sprocket divided by the number of teeth on the smaller sprocket, not the sprockets’ diameters.

The maximum recommended ratio for a single reduction is 7:1. In practice, the practical single reduction limit is affect by the minimum size of the small sprocket, the maximum size of the large sprocket, and the need for sufficient wrap on the small sprocket. For ratios larger than 7:1, a double reduction is preferred.