Even though automakers have made big vehicles more fuel-efficient in recent years, the lower upfront cost of a used S.U.V. appeals to some buyers who need something large. For example, the 2013 Ford Explorer with a 4-cylinder EcoBoost engine gets 28 miles per gallon in highway driving, compared with 18 miles per gallon for a 2003 Explorer. But the new Explorer has a sticker price of more than $30,000, while the Kelly Blue Book retail value for the 10-year-old model is about $8,100.