I've been the owner of some stinky cars in my time. I won't give you the complete history, but let's just say that every topic covered in this article is written from personal experience. I've dealt with odors from cigarette smoke to bodily fluids, including, but not limited to, vomit, deer blood, mucus, dog feces, calf scours, and urine of goat. I've had leaks around doors and left my windows open in rain storms. I've forgotten groceries under seats for weeks, and experienced the nauseating realization that a mouse has died somewhere inside my car. It all stinks, but it all can be cleaned up. Air fresheners are nice, but to really get rid of a car smell, you need to get at the source. If I can do it, so can you. You don't have to live with that smell inhabiting your car.
Cigarette smoke really stinks. As a former smoker who polluted many a car with carcinogenic crap, I know that cigarette smoke isn't only stinky, it's also quite sticky in that it forms a resin on surfaces. That resin is called tar, and though it's not the same stuff that's used to pave roads, it does share a couple of similarities. The first step to removing this smell is to stop smoking in the car. Next, open all of the doors and windows on a nice, warm, windy day. Then go over every surface with a 50/50 water and vinegar solution. That should be able to cut through the resin. If you have trouble, add a couple of drops of dish soap to the mix. Allow to dry completely before closing the doors.
Mildew and wet carpet smells are gross. Another common car smell is caused by the fact that our cars drive through all kinds of weather. Car designers do a pretty good job, initially, with keeping the rain out of our cars. But a small bit of damage to a seal or one open window forgotten in a rain storm could mean that you have potential for mold growth. The best way to deal with this situation is to remove floor mats, soak up excess water, extract it from carpets and interior fabrics with a wet/dry vacuum, and set up a fan to keep air moving with all of the doors open. If the mildew has already done its damage, then you are going to have to do some shampooing. You could use a commercial product or the homemade recipe listed in the right sidebar.
Food smells aren't difficult to remove. The wiser among us probably have rules against eating in their cars. First of all, if you're driving, it's dangerous and might be illegal. But secondly, it's almost inevitable that you are going to drip or spill something. If you have cars like mine (not worth the rubber on their tires), this is the least of your worries, but if you are hoping to trade that car in at some point, you are really screwing yourself. Very often food smells go beyond drips and spills. More often it is related to an abundance of fast food wrappers, some of which might contain half-eaten sandwiches, a passed-over french fry, long-languishing lettuce, the gnawed on bones of a not-so-recently-fried chicken. Look under your seats. You need to throw that stuff away! Good grief.
Bodily liquids and other odors. Accidents happen, especially if you are the owner of a leaky Labrador, an intoxicated roommate, or a human child. Bodily liquids like urine, vomit, mucus, blood, and feces are potent pollutants that will sour the interior of any automobile. Besides their potential for infection and unavoidable ick-factor, they are also notoriously difficult to clean. You need to remove any solids and soak that stuff up right away. Absorbent powder is helpful in this kind of situation, but you could use paper towels, kitty litter, and/or baking soda if they are handy. If the liquid has dried, then you will need to scrape up the dried matter, rehydrate with water and vinegar, and extract with a wet/dry vacuum. Also check out enzymatic cleaners in the left side-bar.
New car smells are actually sort of poisonous. I've never personally experienced the scent of a new car and I probably never will smell a new car that I actually own, but I know it is not something to be idolized in the form of a scented tree. Those so called "new car smells" are called Volatile Organic Compounds (VOC), and they are the same smells common in refinished rooms and new houses. Carpet glue, chemicals from making vinyl, drying paints, and hardening plastics all do something called off-gassing. You really don't want to breathe that stuff; it's one of the things that causes sick building syndrome, and it is potentially carcinogenic. If you think you are having a problem, only time will solve it, but if I were you, I would lodge a complaint with the company.
Petroleum and Exhaust Fumes
If you start to notice a faint scent of gasoline in your car and you haven't been spilling it on yourself, you should really bring your car into the mechanic as soon as you can. You probably have a leak somewhere in your fuel system. Obviously a gas leak has the potential to be dangerous in that it could cause your car to explode and kill you and everyone you love in a fiery ball of death. But it's also polluting, albeit on a small scale, and a waste of gasoline—which never seems to get cheaper. Petroleum fumes can also give you headaches and cause dizziness. So, too, can the fumes that leak from an old or damaged exhaust pipe. Exhaust fumes contain CO, or carbon monoxide, which can even kill you in high enough concentrations. Just bring it in and get it fixed!