Again, thanks for the help. The voltage that I get is positive is just the wiring of the multimeter.
The car was with the ignition key on and with the engine off, so I think this is why my multimeter reads a lower value than 12v. (With the ignition on but engine off the voltage of the battery drops a lit of bit) Plus, it is an old multimeter and it is not very accurate.
I can do the test with the 5v of Arduino, but in my project, I need to use the 12v since the speed must also be read from the ECU of the car which operates at 12v
When I did the test the car was not running as said, I was pushing it very slowly in my garage.
I am stuck figuring out how I can lower the 12v pulse signal to a 5v pulse signal for the Arduino without disturbing the signal