You need to go by posted EPA mileage of each vehicles not what mileage you are getting in your car. If you are 30% over EPA it's a good change the new car will be 30% over too if you are applying the same mods and techniques.
So if the posted EPA value of the new car is higher then the posted EPA of the LeSabre then apples to apples the higher one wins. Also new cars are usually a lot better on emissions even if the mileage in not that much higher.
Having driven beaters my whole life when I rent a car I always come away with a strong feeling of " I could get used to that"