You might adopt a rule of thumb. For example, let’s say that you project that a new car will require a $400 monthly payment. 12 months of payments are equal to $4,800 per year. You can use that as a barometer of annual repair expenses on your current car.
If you have, say, three repairs on your car during the year at a total cost of $1,500, you might decide that—based on the car payment barometer—it’s worth keeping your old car. If, on the other hand, you have five repairs at a cost of $3,500, you might decide that it’s close enough to the cost of the payments on a new car to make buying a new car worth it.
But there are other situations where the type of repairs might also be a factor.
For example, let’s say that your 10-year-old car needs a new transmission, at a cost of $2,500. That’s a big cost, but it also will replace a major component in your car. By doing so, you may decide that the car has a “new lease on life,” and decide to keep it a few years longer.
In addition, when you make that kind of new investment in your old car, you might want to recover some of the cost by keeping the car longer. This will be especially true if the car is reliable, and doesn’t require ongoing repairs.
Still another repair-related factor is your own ability. If you are adept at repairing cars, holding onto an older car longer will be less expensive to do. On the other hand, if you know nothing about auto mechanics, and must go to a repair shop for all repairs, the cost is can be much higher.
Source: https://www.moneyunder30.com/when-to-buy-a-new-car