According to a recent Reuters article using a model developed by the Argonne National Laboratory in Chicago, the point at which an EV’s carbon footprint meets and begins to fall below that of a comparable gas vehicle in the United States is at around 13,500 miles. So after an average year of driving, the EV is better for the environment. Beyond that, the gap widens.
As for keeping your current car:
As far as manufacturing goes, the old car is already built, so let’s give it a pass regarding its manufacturing carbon footprint. According to a 2015 Union of Concerned Scientists report, a full-size long-range (265 miles) vehicle had a carbon footprint of about six tons, or 12,000 pounds.
In two years, the EV will have caught up to the used car in terms of ecological footprint. After that, as with new gas cars, an EV surpasses it in efficiency for its entire life cycle.