The reason you would overdrive the car (smaller pulley up front) is if you are running foam tires and have smaller diameter fronts. You would do this to either compensate for higher wear in the rear or if you have too much grip and need to cut the tires down to reduce traction rolling. You're best off using the same sized pulley for front and rear. If you have the David Jun car, you actually have a mid one-way and diffs in front and rear. Well, unless the guy replaced the front diff with a one-way. The mid one-way reduces the drag when you are coasting, but give you the same lack of braking effect as a front one-way. It's not a bad thing to have depending on the track you're on. I run the mid one-way in my car because I like being able to swing the backend around in rally. It makes the car drift really nicely.