It’s obvious that used cars cost less than new ones. But to really see how much you can save, you have to look at overall owner costs, which include depreciation, fuel usage, insurance, interest on financing, maintenance and repairs, and sales tax.

The real key to used-car savings is depreciation, or how much value a car loses over time. On average, depreciation accounts for a whopping 45 percent of a new vehicle’s owner costs over the first five years, with the steepest drop in value coming in the first year.

By contrast, a three-year-old vehicle has already taken its biggest hit in resale value. Its depreciation accounts for only about 25 percent of its five-year owner costs. Big depreciation for new cars means lower prices for used cars, which in turn mean lower finance charges and sales tax. When combined, that can cut overall owner costs dramatically.

But we also found that some popular models, such as the Mini Cooper and the Toyota Prius, don’t depreciate much. So buying a used version of those models results in modest savings.

We included depreciation in our owner-cost estimates because we assume the vehicle will be sold or traded in after five years, which is a common scenario. That said, the longer you keep a car after the loan payments are through, the more value you get from it. So you could save even more (see Make your car last 200,000 miles).