[MrForbes]

1. Take it apart.

2. Wipe off grease from parts.

3. Count gear teeth.

4. Multiply and divide as necessary to find ratio.

Notice that some of the gears have 3/8" hex holes in them, us lucky Yanks can buy hex shaped aluminum shaft that size to make shafts to use those gears in our own housings (for using one or two stages of the FP gears). You folks in metric areas might have trouble finding parts like that.

edit: Eric and I think alike