How to determine Gear Ratios

[orbeamlb]

I have a 1991 Mercury 200 XRi and noted there are two different gear ratio lower units. How do I identify which gear ratio my motor has?

[patchesII]

Mark the position of the flywheel and propshaft using a Sharpie or whatever. Put the motor in forward gear and by hand turn the flywheel clockwise exactly 4 revolutions. If the propshaft turned exactly 2 rotations its a 2:1. If it turned a little over 2(2.13) its a 1.87. You can do the same to determine the difference between a 1.87 and 1.78 but you'll probably have to turn the flywheel more than 4 revolutions to get some separation between them. Your case is most likely a 1.87

[HStream1]

Erik got it right as usual. I do it a little different and it works just as well.

Think of your flywheel like a clock face after you mark it a 12 O clock. Always starting at 12 o clock.

2:1 is a no brainer, rotate your flywheel two revolutions back to 12. Prop should rotate one complete revolution.

1.87:1 is 1 & 7/8 revolution or one complete rotation then stopping at 11 o clock = one prop rotation.

1.7:1 is 1 & 3/4 revolution or one complete revolution then stopping at 9 o clock = one prop revolution.

Like Erik said you're most likely 1.87.

[orbeamlb]

Erik got it right as usual. I do it a little different and it works just as well.

Think of your flywheel like a clock face after you mark it a 12 O clock. Always starting at 12 o clock.

2:1 is a no brainer, rotate your flywheel two revolutions back to 12. Prop should rotate one complete revolution.

1.87:1 is 1 & 7/8 revolution or one complete rotation then stopping at 11 o clock = one prop rotation.

1.7:1 is 1 & 3/4 revolution or one complete revolution then stopping at 9 o clock = one prop revolution.

Like Erik said you're most likely 1.87.

Thanks Erik and Bud. I'll give it a whirl tomorrow. Working with Power Tech Props to get the best prop for my MasterCraft Prostar 200 Barefoot boat and the the Tech mentioned something about what lower unit (gear ratio and something about a short shaft?).