I installed a Ryca Tachometer on my 2007 S40 and it has
worked well so I thought I would share the installation details.







I mounted it in a location that I thought looked somewhat "stock"
and used rubber grommets to isolate it from harsh vibration.
Refer to the pictures that follow for clarification of the parts and instructions.

Parts List
   Tachometer from Ryca
   Aluminum Angle  -   1/16" x 2" x 4-1/4" Long
   Stainless Steel U-Bolts - 1 inch  (2)
   Rubber Grommets with I.D. that fits over nuts on tach mounting studs  (2)
   Washers with O.D. approximately same as grommets  (2)
   Krylon Clear Satin Finish (spray can)

Tools Needed
   Hack saw
   Drill and Bits
   File
   Wet-or-dry sandpaper, 400 grit and 600 grit

Instructions

Cut aluminum angle to length of 4-1/4"
Bend angle from 90 degrees down to 70 degrees
Cut one leg of angle from 2 inches wide down to 1 inch wide
Cut and shape 2 inch leg as shown in photos or to your own preference
In the 2 inch leg drill two holes for the grommets and one hole for the cable
In the 1 inch leg drill two clearance holes for the u-bolts spaced 2-3/4 inches on center
File and sand all edges of bracket smooth
Wet sand bracket with 400 grit, then 600 grit to satin finish
Coat bracket with Krylon Clear Satin Spray Finish
Check fit of u-bolts on risers and bend open slightly to fit if necessary
Cut off one leg of each u-bolt as shown in photos then file and sand smooth
Cut threaded leg of each u-bolt to proper length by test fitting on risers with bracket
Mount tachometer and bracket assembly on risers
Connect wires to coil and horn as shown
Note: Connect both the Blue and Red wires to the positive (+) horn wire (Orange/Black)

luckylouie:
As I recall I was able to slip the two wires into the spade
connector on the horn. I am not certain about that, and unfortunately
my S40 is at our coach in North Carolina.
An alternative is to get an in-line splice connector at an auto parts store.
You can identify the proper horn wire with a 12 volt test light or voltmeter.
Whichever wire has 12 volts with the ignition switch on is the correct one.

Here is a link to the RYCA website....and it shows that the tach needs 1 spark for each revolution.  A 2 cycle single cylinder, a 2 cylinder 4 stroke bike with a distributor....or a single cylinder 4 stroke bike without a distributor and one wasted spark in the engine cycle meets that requirement.

You need to just be sure what you are ordering will work with one spark for each revolution of the crankshaft.

http://shop.rycamotors.com/mini_tachometer.html

Why would the output shaft be geared down? the transmission provides gear reduction
Which is my way of saying I don't think so, but i might be wrong

I made this mount out of 1/4" X 1" aluminum flat bar. I bent it with a hammer and a vice then sanded and filed them to match the rest of the bike. I screwed them to the bottom of the triple clamp. The Ryca tach works perfectly.

I used Todd's method. I couldn't get the exact dimensions for the materials, but I worked it out  

The perfect spot! Looks great!

Here is what the brackets look like. A little filing, sanding and polishing gives you a sleek looking mount. Cost was $12 for the aluminum and a couple dollars for the bolts.