A fresh set of alloys changes the look of your car instantly, but dropping $1600 at once can be a tough pill to swallow. The best time to strike is during the spring or fall when shops are busy with seasonal tire changeovers and often offer deals to keep their bays moving.
You want to negotiate the price of the wheels and installation separately before you pull out any payment method. Get your final price locked in writing first, then use your payment plan to manage the cash flow over the coming months.
That 18-month window is a deadline, not free money. If you have any balance left after those months, the interest will kick in and likely be much higher than you want to pay, so stay disciplined.