Bobert- Your math works if you're an insurance company...not so if you're a patient.
Try this one: If your car gets 15 mpg and you swap it for one getting 30 mpg, over the course of driving 15,000 miles you'd save 500 gallons. If you want to swap again, what gas mileage would your newest car have to have to provide the same yearly savings compared to car #2?