Don't forget also your lightened flywheel will make the car accelerate quicker as there is less weight to get turning, however when on the run down, it will stop quicker, as there is less weight to keep it rotating. These two figures should balance themselves out though to not affect the final reading.

Think of it as a roundabout in a park. When it's empty it will be easy to get spinning, but let go and it will stop quickly, but put a load of fat kids on it and it will take a lot of effort to get going to the same speed, but let go and it will take much more time to come to a stop!