[Zhangy]

Try it out yourself. After spare tire is removed, some noticeable difference:
-car is a little bouncier, you feel bump oscillations more
-while stopping with your breaks, you roll forward much less... you feel like you can stop on the spot
-acceleration feels "easier"
-steering wheel feels harder to turn at corners

After adding 20gallons of gasoline in tank, some noticeable difference:
-car's bounce is improved, bump oscillations is decreased
-steering wheel feels much easier to turn at corners
-breaking you feel rolling increases by a little bit