well i hooked up an leg to the signal from the car and the ground of the car and it powered it with a resistor, the output is 5v from the car and i dont know what the max frequency is... how do you think i could find that out? and ive been seeing things about useing interrupts but i have no idea how to use them and how i would set up the code...
What Arduino are you using? If it's the UNO then it has 2x external interrupt pins (pins 2 & 3) have a look at the reference
pages for how it works.
To find out the max frequency you need the datasheet for the sensor.
The interrupt will 'interrupt' the main program when it gets triggered by a pulse and when the interrupt(s) have been serviced the code execution in the main program will continue from where it was.
The two main methods for reading frequency using interrupts is to either count how many happen over a known time period or count the time between each interrupt. The former is probably more accurate but as your only using this to light 5 LED's the second should be fine. The principle of the interrupt routing could be...
Read processor time
subtract from last interrupt time
store the time difference or an extrapolated frequency value.