they fit either way....do you want to gear up or gear down?
The gears are a set.
The lower shaft is the input.

If you put the smaller gear on the input, you are gearing
lower (numerically higher). The ratio of the two gears
is multiplied by the ring and pinion ratio.

A lot of installations run a 1:1 ratio on the the gears
if the ring and pinion is a 3:73.

Mike