If you've recently purchased a gently-used vehicle and find yourself dismayed every time you fill up at the pump, you may already be dreaming of a new, more fuel-efficient vehicle. However, churning through cars over a brief period of time can be expensive, particularly when cheaper alternatives exist. There are several relatively inexpensive modifications you can perform to make your car lighter, faster, or just more economical. Read on to learn more about the changes you may want to make to your new-to-you vehicle. 

Buy a lighter driveshaft 

One of the key factors in any vehicle's fuel efficiency is its weight. Many manufacturers are now constructing even heavy-duty vehicles from lighter aluminum as an alternative to heavy steel. Fortunately, a custom lightweight driveshaft is one way you can significantly reduce your vehicle's weight without replacing body panels or making other major changes. 

Most driveshafts are fabricated from steel or a heavy alloy to give them the strength to transfer power from the engine to all four tires. However, the lightweight aluminum driveshafts often used for racing can serve a dual purpose by rendering your car much lighter and more aerodynamic. 

To save money on this modification, you may opt to shop around for used driveshafts at local custom shops or scrapyards. You should also be able to fetch a good price for your used driveshaft due to the scrap value of steel and the demand for certain types of driveshafts. You can also have your own custom shaft made if you want. One company that might help you get what you need is Jons Shafts and Stuff.

Clean out your trunk

Another contributor to the weight your vehicle carries is hidden in your trunk. If you keep a spare tire and jack in your car, you could be adding nearly the weight of an entire extra passenger each time you get behind the wheel. Often, removing this weight and purchasing a tire repair kit, a can of pressurized tire inflator, or a roadside assistance club membership, can be cheaper and easier than changing a spare yourself. 

Look into fuel alternatives

If your car is a diesel model, you may be able to save money on fuel by slowly converting to a biodiesel mix derived from recycled heating or vegetable oil. Most modern diesel engines are capable of running on a mixture of pure diesel and biodiesel, and can even be converted to run on only biodiesel with a few minor modifications. Because biodiesel can be more corrosive than pure diesel to rubber tubes and seals, you'll need to replace these with more durable plastics and metals before switching to all biodiesel -- however, this ultra-clean fuel can help your engine and particulate filters last longer.