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BREATH OF FRESH AIR

Scent is usually the first thing people notice when entering an unfamiliar car. And for some drivers, that's bad news. A smelly car can end up costing hundreds of dollars in detailing costs and even lose value over time. Odors embed into upholstery and carpets, until that "new-car smell" becomes a used-car funk. Some odors, such as those from smoking or leaving food in the car, are preventable. But others, such as spilled milk from a toddler on a sticky summer day, are harder to avoid. The good news: There's hope for smelly cars. With these methods, keeping a car smelling fresh is remarkably cheap and easy. All it takes are a few household items and some imagination.

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DRYER SHEETS

One of the most effective ways to keep a car smelling fresh is stunningly easy: Grab about 10 dryer sheets, put them in a plastic bag with a few holes in it, and toss it under a seat. A box of 100 dryer sheets is about $5 and can easily last a year. Meanwhile, a pair of Febreze car fresheners costs about the same but lasts only 30 days. Some dryer sheets are more fragrant than others, so adjust for strength by simply using fewer sheets or poking fewer holes in the plastic bag.

Related: 20 Cars That Are Cheapest to Repair

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PLASTIC MATS

With plastic mats lining the car's carpet, shoes caked with mud (or worse) will never again be a problem. WeatherTech makes custom mats for a variety of car models that start about $100, and generic mats can be purchased for less. Plastic mats are great for drivers with children or pets, because they're easy to clean and don't absorb odors. Plus, the mats eliminate the costs of shampooing and conditioning car carpets.

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CAR SEAT PROTECTOR

It's no secret that some of the worst car smells come from kids. Be sure to keep clean towels handy, allow kids to drink only from cups with lids, and avoid messy snacks. But sometimes there's no helping a spilled-milk catastrophe. That's why investing in a car seat protector -- essentially a fitted mat for kids to sit on -- can end up saving money and time spent cleaning. Basic models start at about $10.

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BAKING SODA

When faced with a particularly strong odor, or one that seems to come from everywhere in the car, baking soda is one of the cheapest and most effective tools. First, make sure everything in the car is dry. Sprinkle baking soda all over the affected areas and let it sit for a few hours. Then simply vacuum all the baking soda. Consider taking the carpet mats out of the car to sit for a couple hours, as well.

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AIR CONDITIONER ODOR TREATMENT SPRAY

Over time, bacteria and mold can accumulate in the ducts of a car's air conditioning system, resulting in some truly funky smells. Fortunately, these odors can be eliminated with a can of air conditioner odor treatment spray for about $8. To use it, turn the air conditioning on high with the recirculation button off. Spray the treatment into the exterior air intakes at the base of the windshield and wait 10 minutes. After that, turn on the recirculation button and spray the treatment into the interior vents, again waiting about 10 minutes.

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KEEPING THE EVAPORATOR CORE DRY

The dark, damp ducts of a car's air conditioning system are a perfect environment for mildew. It commonly forms on the evaporator core and its fins, both of which can collect moisture during use. To dry out the system and prevent mildew from forming, run the car's blower with the air conditioner off for about two minutes before turning off the engine. It requires some discipline, sure, but it beats that sour mildew smell.

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FRESH HERBS

Why turn to chemical sprays to freshen up a car? Simply buy a bit of fresh basil for a couple bucks, lay it on some newspaper, and set it in the back of the car -- preferably under the back glass where it will receive sunlight and dry out. Other aromatic herbs such as thyme, rosemary, and mint work well, too.

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UNLIT CANDLES

A super-easy way to freshen up a car is to grab an unlit candle and stow it under a car seat. This method works only in summer, however, or when the car's interior reaches a temperature high enough to soften the candle and release some of the scent.

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DECLUTTERING

Odors often come not from spills but from the things people store in their cars -- gym clothes, extra shoes, etc. Treating cars like storage bins not only makes them smell but causes unnecessary clutter. It can even cost drivers extra gas money -- the Environmental Protection Agency says each additional 100 pounds in a car can reduce its fuel economy by 1 percent. So keep all that random stuff where it belongs -- at home.

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WHITE VINEGAR

The acidic properties of vinegar make it a natural cleaner and deodorizer for car interiors. A gallon of white vinegar costs about $4 at grocery stores and general merchandise retailers. Mix equal parts white vinegar and water in an empty spray bottle and use on pretty much any part of the car. Beyond reducing odors, white vinegar is also an easy-to-use cleaner, in part because it's nearly impossible to go overboard (beyond the temporary smell). It can clean upholstery, remove baked-on bugs and bumper stickers, and wash hard surfaces.

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PET SEAT COVER

No matter how cute they are, pets are the arch nemeses of the clean car. They scuff, slobber, shed, and sometimes do much, much worse. One foolproof way to protect against costly pet messes is to use a pet seat cover. Unlike a child seat cover, a pet seat cover attaches to both the front and back headrests, creating a canopy in the backseat that blocks pet access to the car floor. With several versions priced at about $20, a pet seat cover is a simple and fairly inexpensive way to keep a car clean.

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COFFEE GROUNDS

This cheap method for fighting odor sounds strange, but it might just be the perfect deodorizer for coffee drinkers. Fill a tin with about 8 ounces of coffee beans and poke holes in the top. Then simply leave it in the car overnight. The oils from the coffee beans absorb and eliminate odors, rather than mask them. Coffee grounds also work, but beans tend to be less messy.