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
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
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
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
*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.