When choosing a flywheel take note that a heavy flywheel stores more energy as it spins than a light flywheel. If you have a heavy car combination that requires more power (or inertia) to make it launch without bogging the engine, you will want a heavier flywheel. Remember that a heavier flywheel will be slower to accelerate through the gears where a lighter flywheel creates less inertia at launch but will accelerate faster through the gears.