It's possible, but when you pass an iron disc through a magnetic field, voltage is created. How do you think a generator works. Why not harness that energy while you slow your car, you know, you could drive an electric generator off of the output shaft of your transmission. When you're ready to slow down, use the generator to charge a bank of batteries. The load created by the charging will slow the car some. That energy can be reused to accelerate the car, Toyta Hybrid style. The rest of the load could be generated by creating a short circuit across the generator and controlling its resistance.