Question about speed sensor

I'm working on an arduino run speedometer, and while I've got one functioning, I'm not happy with how it operates. what is the golden number for the speed sensor? Meaning, how many pulses per mile does the 3rd gen Prius have? I understand that the speed sensor uses a gear and a magnetic sensor that senses when a "tooth" on the gear passes by the sensor therefor having a given number of "detections" in the form of pulses on a the speed sensor wire per revolution and a given number of revolutions per mile. Therefor a number of pulses per mile.