How much you take out of your paycheck each month has nothing to do with the tax credit. Lets say you have had 11,000 taken from taxes and you owe the government 7,000. You would get back only 7,000 instead of the 7500 AND get back the 4,000 you overpaid in taxes for a total 11,000.
You can ONLY use the credit if it is a NEW car. And you can take the Credit if you Itemize or take the standard deduction.
