Fuel economy is doubly virtuous. It makes cars cheaper to run. And, by reducing the amount of carbon dioxide (CO_2) they release into the atmosphere, it lessens the greenhouse effect. The obvious place to look for fuel economy is in the engine: hence the current interest in such novel power packs as fuel cells and petrol/electric hybrids. But there is also a lower-tech way of reducing the amount of fuel that cars bum. This is to make them lighter.
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