Buying a Vehicle
The 20/4/10 rule
When buying a car, you should put down at least 20 percent. You should finance the car for no more than four years and spend no more than ten percent of your gross income on transportation costs.
Why It Works: It keeps you from buying more vehicle than you can afford. It also takes your ongoing budget into consideration by calculating total transportation costs. These costs include not only your car payment, but also your gas and insurance, which can vary by vehicle type.
When It Doesn't: Depending on your situation, these numbers might not be realistic for you. For example, you may have a long, gas-guzzling commute at a low-paying job, making your transportation costs more than 10 percent. On the other hand, if you've got the cash, you might choose to pay for your car upfront rather than take on a loan with interest. In that case, the rule wouldn't apply to you.
The 10-Year Rule
This rule has to do with the decision to buy new vs. used. If you want to maximize your car's value, you should either buy used, or buy new and drive the car for ten years.
Why It Works: The rule minimizes your depreciation hit. If you buy a car that's a few years old, the depreciation will have already been sucked out of the vehicle. If you buy a new car and keep it for a decade, you'll have optimized its value and the depreciation won't matter as much.
When It Doesn't: Rather than put a timeline on it, some people prefer to drive their cars into the ground, whether they're new or used. The rule also doesn't consider the type of car. Some cars may last well beyond ten years; some may become a financial and maintenance headache after six years. Make sure your maintenance costs are worth it once you get near the end of the car's life.
You definitely don't want to spend more than you can end up paying for your car. And these rules help make sure you're on track with that. But research is important in considering all the variables. Edmund's has an affordability calculator, and we've written about the four questions you should ask when deciding on a new vs. used vehicle. These are also merely starting points, but they're a little more tailored to individual circumstances.