So I'm trying to build a digital RPM gauge. I'm reading pulses from the RPM #18 tachometer wire from the back of the gauge panel.
Can anyone tell me how many pulses would be an RPM?

Thanks

tach's were used back when they had points. One every time a plug fired. It would take 2 "R's" to get all 8 firing events in on a 8. Modern cars may have that info on a data bus. I missed that bus a long time ago. The tach just counts the firing events. the switch on the back of the after market tachs sets it for how many is one "R".
So, 2 for a 4............3 for a 6.............and 4 for a 8.

You question is a bit of a non-sequitur...

Pulses per RPM is not physically possible

RPM is a rate, Revolutions per minute.

What you are really asking is how many pulses per second equals RPM. Pulses per second is known by the more familiar Hertz

for 60 HZ (3600 pulses per minute)

V8 - 900 RPM
V6 - 1200 RPM
L4 - 1800

In other words just as FFIEROFRED said,

for a 4 stroke engine the number of pulses is per revolution = N/2 where N = number of cylinders.

Thanks so much for replying.. I have figured it all out and I'm looking forward to showing off my work.. It's been fun getting to this point.
Thanks again..

Arduino powered?

I'm not sure how high the voltage spikes might be on the tach wire. You should consider using a zenier diode to limit the votage spikes.

[This message has been edited by phonedawgz (edited 11-29-2012).]