Interfacing of 8V hall effect sensor with microcontroller to measure RPM
As the purpose of reading the signal at the microcontroller is to obtain the pulse rate, and not actual detailed waveforms, all that is needed is for the signal at the GPIO to be near zero for 0 Volt input, and reasonably higher than the GPIO's "High" threshold for ~8 Volt input. 

The schematic below will provide a 0 to ~4.7 Volt signal that can be used for the GPIO. Feel free to use other combinations of R1 and R2 to tweak the desired voltage. The zener protects the GPIO against spikes from the ECU, if any.







simulate this circuit &ndash; Schematic created using CircuitLab