The problem here is that the motor and generator doesn't have 100% efficiency.
Plus, the front wheels are placed under more load because the back wheels are using the extra power for power generation. In other words, the theory seems flawless, but in reality, it's a bit counter intuitive.
What you can do is to have a regenerative motor. Meaning, the motor acts as a generator when the car slows down, which is the technology used in hybrids and electric cars/motorcycles these days.
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