Not sure I understand the logic, so let me work this out. Let us say you buy a car at $40,000 + $1000 for transportation + dealer stuff + say $3280 for sales taxes @8%. Your total out of door cost is $44280. You can now reduce your income taxes by $7500 when you file income taxes in 2020. So this needs your tax liability for 2019 (after all the other deductions and personal allowance) to be more than $7500. So your net cost is $44280-$7500,= $36,780.
I think your assumption was that your out of the door cost is $40,000-$7500=$32,500 +$1000+$2680 (sales tax)=$36180. So you are pretty close, that if they allowed you to take the credit in advance, you would have saved 8% of $7500, which is $600. (Your Mileage will vary, as actual costs, taxes etc vary, but $500 to $ 600 is about the ball park).
However, there is another cost, the time value of money. If you buy the car in April 2019 and get your refund in April 2020, you have lost potential growth at say 4% for a year, which is $300. However if you bought the car on December 31st and got your refund by Jan 31st, you have an opportunity cost of $25. Of course, if you can manage your withholding so that you contribute less per month, you can minimize the impact, but those become hypothetical calculations for most. To put it simply, you have to fork out the money upfront and you will get it back after say 1 to 15 months, based on when you buy and when you get your refund.
