Well, well, well!. The crank is going around twice for every revolution of the mag, so I would think you need a 2:1 tach. Is the tach of the magnetic or chronometric variety. I'm thinking the former because if it were chronometric you would expect either a 2:1 for mag driven or a 3:1 (as Trev says) for oil pump driven. I'm not a tachometer expert, but it seems unlikely Smith's would go far afield to make a 4:1 chronometric that did not work with bikes of the day. Maybe another member knows of a bike that uses a 4:1, but it seems it was not n A7 or A10.What does it say on the face? Speaking of faces, I'm prepared for egg on mine if I am way off here.
Richard L.