Tachometer Malfunction: Description
WARNING: This page is about a different car, the 2008 Lexus IS F. However, it is still accessible from the selected car via links, so may be relevant.
The meter CPU receives engine speed signals from the ECM in this circuit. The ECM transmits engine speed signals as pulses to the meter CPU. The meter CPU calculates the engine speed converting 4 pulses to 2 revolutions.