Today I fitted a Ryca Universal Mini Tachometer (came with handlebar clamp) to my 2015 S40. The mini clamp will not fit in the middle of the handlebar as it is double thickness on the S40 with the drag bar. No biggie, as I have a GPS there, so I fitted it on the left hand side of the tree.

I found different methods of hooking it up both from Ryca and on this forum. The Ryca instructions (sheet of paper in the Tachometer box) are: Green wire to negative terminal of the coil, black wire to ignition "on", blue wire to lamp on, red wire to ignition "on".

A favorite grounding method seems to be fitting the black wire under one of the screw holding the coil. Despite using the correct Japanese screwdriver, and then an impact screwdriver with a Philips #2 impact bit, I could not get the screws to turn, and I did not want to start drilling... so I extended the black wire and placed it under the screw holding the reflector on the left side of the bike.

For the other wires:

Green wire to negative coil terminal (B/Y loom wire for 2015 S40)
Blue and red together to positive coil terminal (O/W loom wire)

It works!

A little off topic:

I found a wiring diagram on ThumperTalk that may adapt a "normal" tach to our CDI ignition. Anyone have a "regular" tach lying around and want to be the guinea pig?