In a four stroke engine, does the tachometer (or the ECU) measure and
report crankshaft rpm or engine rpm? Obviously, the crankshaft rotates
twice for every time the engine goes through a full cycle. So a
reading of 1750 rpm could mean the crankshaft is rotating 1750 times
per minute and the engine is going through 875 cycles per minute, or
that the engine is going through 1750 cycles per minute and the
crankshaft is rotating 3500 times per minute.

Since, I believe, the trigger comes from the ignition circuit (in a
spark engine) I'm inclined to think the reading is engine cycles per
minute. But various places I've looked on the web imply (but don't
specifically say) that it's crankshaft revolutions per minute.

Hopefully someone here can clear up my confusion.

Thanks,
Rob