I recently purchased the MSPEC tacho and installed it in my race car... That's where the problems started. I'll copy and paste part of my email to save me retyping:
"It has the usual wires, Red(Ignition Switch), Black (Earth) and Green (Signal)
The wiring diagram suggests there is a yellow wire for permanent 12 volt +... It doesn't
have a yellow wire. What it does have is what appears to be an inline fuse holder full of
small round batteries. This same battery pod swaps the colours of the red/black wires and
exits to a small two pronged plug. Your wiring diagram makes no mention of this assembly.
Luckily I rewired the factory tacho and managed to compete with that. Using your tacho,
on 4 cyl setting showed the motor idles 500rpm lower than it actually does and the low
reading seemed to get worse as the revs increased right up to 7,000 rpm.
The car in question is a RX7 rotary and it's accepted practice to use the 4 cylinder
setting on after market tachometers to obtain an accurate reading."
Thanks 
