You've probably clicked on this link to tell me that you can't do it.
I will say that you can, and it'll possibly save you $50
I didn't want to have to order a tach for my motor, a 1973 65hp 3cylinder Evinrude.
Step 1: Find Out How Many Pole or Pulses You Motor Gives Off Through the Tach
My motor has a 12 pole stator, creating 6 pulses per revolution.
Check with your local dealer.
Unlike outboards, which run off of the stator, Automobile tachs run of the distributor on older vehicles. On automobiles, 4 cylinders send 2 pulses, 6 cylinders send 3 pulses, and 8 cylinders send 4 pulses per rotation.
Step 2: Buy and Disasemble the Tach
I purchased a cheap $30 Sunpro tach from Autozone that has a selection for
4, 6, 8 cylinder motors.
I disassembled the tach, by removing 2 screw from the back and poping off the front beezel with a flat head screwdriver
Step 3: Remove and Replace Resistor With Proper Rating
the IC in the tach takes a pulse reading in relation to the resistance of its control pin.
That being said I replaced the 10 cent resistor for the 6 cylinder position to 147k ohms(2 resistors, 1- 47K and 1- 100K) causing the gauge to read 6 pulses a revolution, and giving me an accurate gauge reading.
1Pulse = 623.3K
2P = 382K
3P = 258.7K
4P = 195.7K
5P = 163.5K
6P = 147.06K
7P = 138.6K
8P = 134.4K
9P = 132.24K
10P = 131.14K
Step 4: Completion
I hope this is helpful and if you have any question let me know.
*note: 6P was tested with an oscilloscope, 1, 5, are calculations only, 7-10 are rough calculations. RPM accuracy is based on you doing your own math and the accuracy of your resistor/s
you might want to use a variable resistor to fine tune.
1/4 watt sould be fine, but use 1/2 watt resistors if you want.
Let me know what you think.