What is car allowance rebate?

"What is the amount of the credit?

The amount of the credit is $3,500 or $4,500, and generally depends on the type of vehicle you purchase and the difference in fuel economy between the purchased vehicle and the trade-in vehicle. Different requirements apply for work trucks.
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"Is the credit subject to being taxed as income to the consumers that participate in the program?

The CARS Act expressly provides that the credit is not income for the consumer.
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see details at Gov website