Tachometer Drive Unit

[Patrick S.]

My rebuilt tachometer indicates about 25% over the engine speed. I checked the pullies on the crank, w/p and generator and they are all the right size. Put on a known good tach and it reads the same, put on a reduction unit (25%) on the back of the drive unit and the reading appears right. Are there more than one size gears in the drive unit?

[Troy P.]

Re: Tachometer Drive Unit

Interesting issue. I also wonder if you have the wrong drive unit in your tach.

[Patrick S.]

Re: Tachometer Drive Unit

I've tried 3 different tachs and they all read the same 25% high.

[Ken A.]

Re: Tachometer Drive Unit

There is no such as a drive unit in the tach.

[Richard P.]

Re: Tachometer Drive Unit

I wonder if the issue is the generator pulley? If it is undersized it would give a higher reading at the tach.

[Patrick S.]

Re: Tachometer Drive Unit

The pulley on the crank and generator are the right diameter and width (groove). Still baffled

[Rod K.]

Re: Tachometer Drive Unit

Believe there were two different pulleys used, one 3-5/8 dia for Corvette and one 3-3/8 dia for low HP and pass applications. Larger dia took a 1/2" wide belt vs 3/8" for low HP. The smaller one wouldn't account for 25% error but would be over by about 7%. Narrow belt in a wide groove would run faster as well.