For every extra 100 pounds inside your car, the overall fuel efficiency is reduced by 2 percent. While that number may sound low, it adds up to a significant amount of wasted fuel (and money) over time. While it's obvious to keep the trunk empty of things like golf clubs and boxes of random stuff, you can also save money by uninstalling little-used car furnishings. If your car has as third row, for example, those spare seats can be taken out and put aside. The heavy extra seats are a cinch to remove, and they can be stored in your garage and reinstalled when you are transporting a crowd.