Why can't the kenetic energy produced by a vehicle be used to power a electric generator?

Why cant the kenetic energy be converted into mechanical energy by way of rear axel to power an electrical generator in order to extend the milage of an electric vehicle?
Update: I am talking about an all electric vehicle. I know there are small generators in gas vehicles that charge a battery. But how about putting a larger generator in an all electric vehicle?
Update 2: Actually not even using the motor to run the Generator. It would be run off the kenetic energy the vehicle is producing by just moving or going down hill. The generator would be using the rear axel which has no power going to it. So the generator would use none of the power going to the motor. Why can't this be... show more Actually not even using the motor to run the Generator. It would be run off the kenetic energy the vehicle is producing by just moving or going down hill. The generator would be using the rear axel which has no power going to it. So the generator would use none of the power going to the motor. Why can't this be possible?
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