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Per pound, steel springs store less than 1% of the energy in a car battery, but they have occasionally been used to start aircraft and small engines. The usual difficulty comes if it does not catch on the first try. Compressed-air starters are lighter overall, effective, but less efficient. However, they can be charged up "for free" by using an air pump as a shock absorber.
It is very unusual for a car to have a combined starter and generator. The alternators are sized to replenish the battery fairly quickly after a cold start while running most of the lights and other accessories. Starter motors are built too light for continuous duty at regular power.
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There is no excuse for a land vehicle to weigh more than its average payload.
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