At only 52 mm, the Mini-Tachometer by Intellitronix is everything you need in a great little package. It is a memory tachometer coupled with RPM activated output and a Blue LED shift light. 
 
Microprocessor controlled. Compatible with 1-2-4-6-8-10-12 cylinder engines. Accurate, super bright, and intelligent all in one compact  package. The future of precision is here.
 
Other features include:

2 1/16 inch diameter, only 1 1/2 inch deep;

Automatic night-dimming;

Uses Push Buttons and Digital Display to program the switch;

RPM Range to 9,900 RPM (four LED Digits--the two on the right only read 0s);

RPM activated switch function:  Choose from Normally Open (turn on an accessory at the desired RPM) or Normally Closed (turn off an accessory at the desired RPM);
            
Functions as a Digital Tachometer with High RPM recall function;

No need to remove cover to program;

Over current protection, Short circuit protection, Over voltage protection;

CAN'T BLOW IT UP!;

Black Face Gauge with a Low Profile Tinted Lens;

Ultra-Thin Design – Gauge is Only 1 Inch in Depth;

Dimmer Wire Allows the Gauge to Dim 50%;