Drive ratio is stamped / written on the outdrive on one of the sides near the top, if I recall, mine is next to the drive serial number.
I agree, you need to verify accuracy of the tach before going much further. Much like a computer, garbage in = garbage out.
Yes.
The drive ratio must be determined AND a known tach must be used or you're wasting your time. It's even possible that someone had that drive rebuilt (and changed the ratio) in the last 17 years and didn't bother to change what is stamped on the side!
Since the boat is on a trailer, you can positively verify the ratio by pulling the spark plugs, placing it in gear (FWD or REV) and have someone turn the crankshaft and count turns (10 or so) .............. Count prop turns, then use your calculator to divide prop-turns into crank turns.
Since you probably have a 3-blade prop you can easily estimate a third of a prop turn................. 0.33 turn= 1 blade..........with a 4-blade, 0.25 turn= 1 blade etc etc.........
When you determine your RPM and drive ratio accurately, you can then investigate engine problems, water-logged hull, dragging the anchor
, forgot to remove the trailer:facepalm: etc!!!