theoretical car question?

i am very eco conscious and i try to get the best gas mileage as i can with a car. Theoretically, would it be better for the environment if i was to buy a less efficient used car which otherwise would be bought by someone who doesn't drive as consciously as me. thereby forcing them to buy a more efficient car which would burn less fuel even with a more lead-footed driver. for example, if i get 20% better gas mileage than a normal driver and i have a car that is rated for 5 gallons per 100 miles (20 mpg) i'll use only 4.16 gallons per 100 miles (24 mpg). however if i have a car that uses 4g/100m (25mpg) i would use 3.33 g/100m. I save more by using the less efficient car. so would this actually be better for the environment?