reduce tach. signal frequency for use with other tach.

I have a Diesel vw jetta that did not come with a tachometer.
However I did aquire a tach. from a gas jetta at the junkyard.

From what I have read the signal provided by the diesel alternator w
terminal is about 25 pulses per revolution while the gas would be
2 pulses per revolution. Ive observed that the gas tach. currently
reads 5800 rpms at Idle.

What can I do to accomplish this? Im thinkin a 4017 chip and maybe a
555 but Im still pretty new to working with these chips so I need some help.

The poor people behind you that must breath the stinking black smoke from your diesel.

If you can condition the input, there are a number of counter / divider chips. As long as you can shape the input to "pulse" them, you can set them up fairly easily as frequency dividers by triggering the tach from one of the high order bits.

An alternative would be a new hall-effect sensor and a magnet (or two) on one of the pulleys - like the water pump or crank - and use that to trigger.

The 4017 is a good start. Maybe look for a binary output counter. Best you'll get is being able to divide by 12 by ANDing the 8 and the 4 output, and wiring them back to the reset (I think)