It's really your call, but since you make four estimated tax payments at various times during the year, and can adjust the amount of each payment, I would make the payments until you have actually bought the car and know with certainty that you are going to get the credit. Then you can adjust or skip the remaining ES payments, depending on the impact of the credit. If you wind up over paying estimated taxes, you'll get a bigger refund next year, and if you don't buy the car, you won't have to pay interest on the underpayment next year.
