If you need to route and wire your 2-1/16″ Nitrous LED Tachometer the information herein might help. The following diagram shows how to route and wire the device. 
To assembly the LED tach, a round 2-1/16″ wide hole with at least 2-1/2″ clearance behind and 18 AWG stranded copper wire are needed for the installation. After disconnecting the battery and move the cylinder selection switch to match the number of cylinder, here is the wiring guide:
- Red wire: to a switched 12 V positive source that turns on/off with ignition switch (use an inline fuse 1Amp/3AG before connecting to 12V ignition system)
- Black wire: to Ground
- White wire: to the dash lights
- Orange wire: to the 12V positive post
- Green wire: to the negative side of the ignition coil
It is noted that if the vehicle has an electronic ignition, please be consulted for the correct tacho signal point since the diagram above is for conventional ignition system.
Further detailed on Routing and Wiring 2-1/16″ Nitrous LED Tachometer can be download in how-to pdf filetype (source: harborfreight.com)
