Mazda RX-8 Workshop Manual page 4110

when inspecting the tachometer, do not allow the engine speed to exceed the allowable
range indication on the tachometer.
1. Connect the M-MDS to the DLC-2.
2. Compare the data monitor item (RPM) with the tachometer indication.
If the tachometer does not operate properly, inspect the PCM and related wiring
harnesses.
Variable red zone
1. Connect the M-MDS to the DLC-2.
2. Measure the engine coolant temperature using the PCM data monitor item "ECT".
3. Verify that the variable red zone is displayed according to the table.
If the PCM and related harnesses do not have any malfunction,
replace the instrument cluster.
Range of variable red zone