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Issue With Improving Fuel Economy improvements to a car's fuel economy
can come about through two basic approaches:

The engine can be made more efficient by adopting new technologies, or the amount of work required by the car's engine can be reduced-

usually by lowering wind resistance, or by making the car smaller and lighter.

The cheapest way to improve fuel economy is to reduce the physical size and the weight of the car, and this is the approach taken by most auto manufactures.

However,making cars smaller has its drawbacks. Perhaps the biggest concern is safety.

Many studies indicate that smaller cars tend to be more dangerous for drivers than larger cars, although this is an area of controversy; 
some  researchers feel that a relationship between vehicle safety and fuel economy has not been established.