I was just wondering if you put a STI cluster in a WRX do you loose your mileage reading? Does it just go to 0?

 

yes, the milage is stored in the cluster

I remember someone saying you need to document the milage when you take off your old cluster and keep it with your paperwork for the car. Thats all that is needed to be done

 

You can also etch it in the door jamb, like the previous owner of my car did.