This is the sensor, http://www.allegromicro.com/Products/Magnetic-Speed-Sensor-ICs/Wheel-Speed-Sensor-ICs/ATS682.aspx
The sensor is for 12V, so I suggest to use it with12V.
The resistor on the low side (between the sensor and ground) seems the best option.
You could bring the voltage down with a voltage divider (two resistors).
Perhaps two resistors of 10k.
An optocoupler is a lot better, it also avoids ground problems.
I didn't read how to calculate the value of the resistor, but with a value of 100 ohm or 470 ohm, it should be okay for most optocouplers.