If a new Prius were placed head-to-head with a used car, would the Prius win? Don't bet on it. Making a Prius consumes 113 million BTUs, according to sustainability engineer Pablo Päster. A single gallon of gas contains about 113,000 Btus, so Toyota's green wonder guzzles the equivalent of 1,000 gallons before it clocks its first mile. A used car, on the other hand, starts with a significant advantage: The first owner has already paid off its carbon debt. Buy a decade-old Toyota Tercel, which gets a respectable 35 mpg, and the Prius will have to drive 100,000 miles to catch up.