I normally trade up when it doesn't pass inspection and costs too much to repair. For my last car that I traded in they wanted 3k worth of work on it. So I just decided to get a new one. I also wait until I have enough to pay cash for a car.

anyone have a good link to share? i.e., my old beater gets 17MPG, but the taxes and insurance are low and the maintenance is low and depreciation has slowed considerably. when do the #s make sense to trade up?