Automobiles are complex machines, and it's nearly impossible for a driver to understand every aspect of their vehicle. However, that doesn't mean that they should propagate the many myths that surround modern cars.

Fuel efficiency has been all the rage lately, as sales of compact and eco-friendly cars spiked sharply last year with the increased price in gas. While buying a car with great fuel efficiency is an excellent way to save some money, drivers might be surprised by just how many myths exists when it comes to filling up the tank.

If you don't have the money to shell out for a new car, one way to improve fuel efficiency is through auto maintenance. Keeping your car in tip-top shape generally helps things run better, and can save you money at the pump. However, drivers need to be realistic about what kind of efficiency increases they can expect.

One popular myth in this vein is that replacing an air filter will improve fuel economy. According to the U.S. Department of Energy, that's simply not the case in modern cars. This myth probably rose out of the fact that on older carbureted engines, replacing the air filter actually did improve fuel economy, by 2 to 6 percent. With fuel-injected engines, however, drivers won't see an increase in their fuel economy. However, they will likely notice an improvement to their acceleration time, so maybe they don't have to press down on the pedal quite as long or hard to get up to speed.

Other maintenance tasks can improve fuel efficiency, however. One thing to keep in mind is that an automaker's claims of fuel efficiency are based on what they recommend for the car. If you ignore the recommended motor oil, for example, you could be hurting your fuel economy. Using a tire pressure sensor is also important, as underinflated tires can cause a 3 percent reduction in fuel economy. Tune-ups in general help with fuel efficiency, and if you end up repairing something like a broken oxygen sensor, your fuel economy can increase dramatically.

Yet fuel efficiency isn't the only myth surrounding gas and the way it works in vehicles. There's also a lot of confusion over the actual act of putting gas into the vehicle. Many drivers, for example, believe that filling up their car with a higher octane fuel will result in a performance increase.

This myth likely comes from the fact that high-performance vehicles require a higher grade of fuel. However, this is because their engines are made to handle and make the most of that fuel. Putting premium gas into your decade-old used car is not going to have any effect. It's best to read your owner's manual to see what the manufacturer recommends, then stick with that.

Some people claim that using a different octane of fuel voids your warranty. It's extremely unlikely that will happen, as it's difficult to prove. However, using the wrong grade of fuel over an extended period of time can cause damage to your engine, so be mindful of that.

Everyone loves saving money at the pump, so for some reason myths about "tricks" that consumers can pull off are prevalent. Some people believe that filling up late at night or early in the morning results in more fuel. This arose out of the fact that colder fuel is denser, resulting in more energy being packed into the same space. However, the weather outside does not affect the fuel's temperature - they sit in underground storage tanks that are usually kept at the same temperature constantly.