You can check your serial number on the old version of Mercury’s website Here
If you do not have a serial number, you can remove the top cap and count the gear teeth.
Bravo 1
1.65:1 = 23 x 30
1.50:1 = 27 x 32
1.36:1 = 27 x 29
Bravo 2
2.20:1 = 23 x 30
2.00:1 = 27 x 32
1.81:1 = 27 x 29
Diesel high HP (these use a special lower unit and are not very common)
1.65:1 = 27 x 32
1.50:1 = 27 x 29
Bravo 3
2.20:1 = 23 x 30
2.00:1 = 27 x 32
1.81:1 = 27 x 29
Diesel high HP (these use a special lower unit and are not very common)
1.65:1 = 27 x 32
1.50:1 = 27 x 32
1.36:1 = 27 x 29