For electric vehicles, the flywheel can be much lighter than the one that came with the gas engine. Gas engines use flywheels to store energy to help with smoother idles, and for starting the car from a dead stop. Gas engines don't produce much torque at low rpms so they need the "help" of a flywheel. Electric motors have great torque at all rpms, so they don't need the flywheel to help accelerate form a dead stop. Since the flywheel stores energy, it takes some of the energy that could be accelerating your car and puts into rotating the flywheel. This is a waste of energy for an electric car. I decided to lighten my flywheel when I had the motor out to fix my transmission. I found a site that developed the equations for calculating the effective weight reduction of the car versus the inertia reduction of the flywheel. From this equation I built a excel spreadsheet that you can download. This will help you understand how much effective weight reduction you can get if you lighten your flywheel. You'll need to know your transmission gear ratios, your rear end gear ratio, and your tire diameter. For my car I was able to remove 7.5 lbs form a 16.5 lb flywheel. This was the equivalent of reducing my car's weight by 171 lbs when in first gear. In 2nd gear the loss was 68 lbs.