Anything you do to improve mileage will reduce your carbon output. Keeping to the speed limit and driving defensively can improve your mileage by more than 30 percent, according to the Department of Energy. Even something as simple as keeping your tires inflated and having your engine tuned up can give you up to a 7 percent bump in m.p.g. — and an average carbon savings of about what you’d save from eating only local foods all year