Much of a car's weight comes from components that have to be sized according to the car's weight. This includes the engine, drivetrain, exhaust, unibody, tires, etc. Weight savings at the design level have a ripple effect of weight reduction.
I ran across this article in the Times' blog, which suggests cutting 400lbs from the unibody of a 3500lb car saves an
additional 125lbs elsewhere in the car.
A Gas Mileage Bonus From Aluminum - Green Blog - NYTimes.com
Quote:
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Originally Posted by New York Times
The American trade association, extrapolating from that study, calculated numbers for a sedan the size of the Ford Fusion, weighing 3,500 pounds. For that car, the so-called body in white weighs about 900 pounds if made from ordinary steel, but only about 500 pounds if made from aluminum.
Making the body lighter means that the engine and brakes can be smaller, too, without affecting performance, so the total weight savings comes to 525 pounds. That would yield 2.7 extra miles per gallon, the group said.
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If the 400lb weight reduction came from outside the unibody, you could reduce the weight of the body as well, so the impact would be much greater.
High curb weight means more than just higher rolling resistance and inertial loads. It also means you have an even larger mismatch between your power requirements during a hill climb vs during cruising. This reduces your BSFC during cruising. Or if you're going hybrid, curb weight means a heavier and MUCH more expensive electric motor.