I am thinking about getting a GTech. How does the GTech measure RPM?

Kind of tough to do without an electrical connection....

 

it plugs into your cigarette lighter and takes a voltage reading while you hold the throttle at certain rpms. Quite accurate actually. I have one and enjoy it.

 

Hi sranger,

It's an amazing technology that the GTech uses to get the rpm signal via the vehicle's electrical system. Know how on older ignition systems with unshielded wires, or how with a really "hot" ignition system you hear alternator "whine" thru the radio? Kinda/sorta along those same lines, everything on the electrical system has some crosstalk or other signal that can be used with appropriate data acquisition technology - and the GTech folks use their technology to grab an rpm single thru the cigarette lighter connection. Very simple, & we use them frequently!

 

I believe it grabs an inductive voltage spike that is imposed on to the DC power by the high voltage ignition when a spark plug is fired and uses a frequency counter to sum and then divide by 8 or 6 or whatever many plugs are used. This would give you 8 pulses for one rpm on an eight cylinder motor or 6 pulses for one rpm on a 6 cylinder motor. Pretty basic but a neat and accurate way to do it.