Gary,

Signal of W-terminal is frequency type (also there exists slight amplitude
changes depending the loading).  Number of pulses/turn being dependent of
qty of poles in your alternator.

Inside of most tachometers this variable frequency is converted to variable
dc voltage and displayed by analog meter.  In �C based meters these pulses
are counted directly and displayed.

Instead playing with speeds and gear ratios I suggest you to go some service
having portable hand held RPM meter and calibrate your own meter to match
its reading.  In my case it didn't cost anything.

Juha Koivisto
88 Vanagon (Caravelle) 1,6TD (old school bus with 9 seats)
TURKU, FINLAND