Bent wheels, axles, bad shocks, broken springs, etc. create engine drag and are unsafe at high traveling speeds. Remove vinyl tops - they cause air drag. Rough surfaces disturb otherwise smooth air flow around a car's body. Remove items that cause wind resistance, such as luggage racks. Remove excess weight from trunk or inside of car - extra tires, back seats, and unnecessary heavy parts.
Extra weight reduces mileage, especially when driving up inclines.
During cold weather watch for icicles frozen to car frame. UN-removed snow and ice cause tremendous wind resistance. Keep tires properly inflated at all times. Keep your car properly tuned for top fuel efficiency. Keep brakes properly adjusted.