>As near as I could tell, whichever electrode had the
>highest conductance got the spark.  When it wore to where another
>electrode had more conductance, the other got the spark.

This is correct. The life of the electrodes is increased by a factor
of 3 (for the 3 electrode plugs) because they trade firing duty. As
one gap wears (as all plug gaps do), the gap increases a little. One
of the other gaps, which isn't wearing, is now smaller, and the spark
jumps across that one. Then that gap wears, and another gap is now
smaller.

So all gaps wear equally, but since they are firing only 1/3 the time
(as compared to a 1-electrode plug), they will last that much (3x)
longer.

Gas mileage may stay constant longer (ie, like you have a new plug
for 3x as long!), but an *increase* in gas mileage over a
single-electrode plug is not possible.

Dave
--
Dave Carpenter

Whatever you wish for me,
May you have twice as much.

"Any sufficiently advanced technology is indistinguishable from
magic." -- Arthur C. Clarke