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In this day of energy conversion and high petroleum costs the optimum consumption of fuel is the goal of all owners of gas and diesel vehicles. The tachometer used with internal combustion engines permits engine revolution monitoring and with proper driver control increased kilometers per litre. The use of electronic automobile tachometers has not been popular because their linearity throughout the necessary RPM range leaves much to be desired due to their temperature instability. This report gives details of the design of a temperature-stable diode-pump automobile tachometer. The use of an unbalanced bridge circuit to interface the diode pump was found to reduce the changes in pump count caused by a temperature gradient. The original work, as reported here, was designed for a hand held instrument to be used by a mechanic. The identical circuitry has been applied to a digital-readout dashboard-mounted automobile tachometer. Diesel engines have used an inductance pick-up from the timing mark groove on the flywheel.