Get the most out of your car

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NEW YORK (CNNMoney.com) -- Here's some advice on how maintaining your car can add miles to every gallon of gas you pump.

If you use conventional motor oil, switch to synthetic oil. Your engine runs more efficiently with synthetic oil according to Lauren Fix, the author of "Lauren Fix's Guide to Loving Your Car." And that means you'll get more fuel efficiency and better performance.

Of course, synthetic oil costs about $5 more than conventional oil, but you can go longer between oil changes. For example, if you change your oil every 5000 miles, with synthetic oil, you can change it for every 7500 miles.

About 17% of the vehicles on the road have gas caps that are either damaged, loose or missing altogether. This causes 147 million gallons of gas to vaporize every year according to the Car Care Council. Make sure you hear two clicks when you tighten your gas cap, and replace your cap if it's cracked or worn.

Pay attention to your car's fuel components. Just one misfiring spark plug costs you two or three miles per gallon. And of course, don't forget about the tire pressure. Remember to check your tire pressure every month on all four tires.

And don't use the tire pressure number that's on the wheel. That particular tire can fit many different cars, says Fix. Instead, get the right tire pressure from the car door. It should be a decal inside the drivers door, inside the glove box, or inside the gas door.

Cars aren't moving off the lots as fast and margins are under pressure, so more and more profits are getting squeezed out of car repairs. You may encounter what's known as upsell. That means you may be persuaded to buy a higher quality or more expensive part. Or, you may be encouraged to have work done on your car that isn't necessary.

One upsell that isn't worth it, according to Fix, is putting nitrogen in your tires. Some mechanics may tell you that it makes your tires last longer and will improve your fuel economy. The truth is that while nitrogen is used in race cars that go 130 miles an hour, for the average person, you'll get a minimal effect. Plus, it'll cost more and it's a way for the station to get you to come back every month. Instead, put regular air in your tires. It's free.

Getting your oil changed is another place where mechanics may try to convince you to replace your tires or a tire alignment. Make sure you choose an auto repair shop that you trust. Don't get caught up in the laundry list of "recommended maintenance." Instead, consult your car's manual and use the Internet to find the best price.