Sam, sounds like an excellent ideal.
Your car with the poorest overall mileage and performance might be selected for the test.

I would set the timing by the piezo method.
Mark the pump and see if it coincides with the highest leading edge of the Milli volt method.
I would not be shocked if it was not a few degrees advanced over the piezo method either with the Milli volt method.

Allowing for aging characteristics or modern fuel perhaps.
This is exactly the kind of information we need.
Can see no reason the two methods cannot be used simultaneously when doing the timing to compare.
As there is no interference between the two systems during application.

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