Why am I having to run with an extrusion multiplier of 60%?

[Dougal1957]

Possibly got the number of teeth on the 2 gears wrong?
and the bad one is 7.75 co sounds like you got a bad one as well but it is very obvious which is which the one with the wider teeth is the good one

[DjDemonD]

The one on the left is the clone 7.75mm quite blunt teeth.
The one on the right is the genuine 7.35mm quite sharp teeth.

As for the gears between motor and the hobbed shaft they are interchangeable so exactly the same number of teeth I presume.

[deckingman]

The really bizarre thing in my case was that most of the issues I was having just went away. It coincided with me having to print a new X carriage, fit and new height sensor and while I was at it, fitted the Duex expansion board. None of those things could have caused a significant change in extrusion though but ever since, I've been running with 90% extrusion which is a lot more sensible than the 60% I originally had to use. There must be some sort of dark magic going on…........