6 months before the warranty expires, or no longer than 80k miles. If you wait any longer, you kill your trade in value. Besides 4-5 years ,and 80k later, technology should have come up with the new bigger better, higher mpg car by then. Every 3-5 years, large improvements are made in the automotive world. So by the time new tech comes in, you should be ready for a new car to stay ahead of the curve, in my opinion.