If the signal is some kind of square wave or pulse that goes between 0 and 12V ( or some other voltage) then someone skilled in electronics should be able to make such an adapter. It would put out 2 pulses for every three that it gets. There was a guy here recently who designed a code reader for pre 95 cars. He could probably program a chip for you. But you would need to tell him the format of the tach signal. I figured that someone somewhere makes such an adapter but I could not find anything with a web search. There is probably such a low need for such a thing. Someone somewhere probably makes one though. Maybe...

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra