There are a lot of reasons to consider pouring some fuel additive into your car's gas tank ... at least, that's what the fuel additive bottles and your mechanic might tell you. According to the labels, different fuel additives can cure a lot of problems with your car. But how much of that is true, and how much is just good old-fashioned marketing magic?
Whether or not a fuel additive will make any difference to your car depends on a couple things: the type of additive you're considering, and whether or not your car actually needs it. If your car could benefit by one of the few additives that actually offers a tangible benefit, then sure, go right ahead. For example, if you have a vintage car (like from the 1970s or before) that was designed to run on leaded gasoline, you probably already know that there are additives on the market that are lead-free, but claim that they can make such cars run better on our modern, unleaded gas. Whether or not these additives actually do what they claim remains to be seen, but they won't hurt [source: Allen].
Drivers of diesel vehicles know that when it gets really cold outside, the diesel fuel can congeal in the tank and in the fuel lines, which can hamper performance somewhat until it all comes up to running temperature. In such cases, there are anti-gelling fuel additives that will help keep the diesel flowing smoothly, especially through the filter, where cold clumps tend to stick. Fuel stabilizers are a good idea when a car is going into storage for an extended period of time, to help the car start up smoothly when it's time to hit the streets again. And if your car is old (like, at least a couple years outside the manufacturer's warranty) it may occasionally benefit from a dose of fuel injector cleaner added to the gas tank.
When not to use a fuel additive? If it's something your car obviously doesn't need at all (like a lead additive on a modern car or a diesel additive on a gas car). If your car is new, or new-ish, everything should still be relatively fresh and running smoothly, and any kind of additive is probably unnecessary. Some people believe that adding automatic transmission fluid to the fuel tank helps lubricate the metal-on-metal contact inside your engine, when in reality, it may actually cause corrosion or other damage. And avoid anything that claims its main benefit is an improvement to the gas mileage or a boost in horsepower. These fuel additives probably won't do any harm, but they don't live up to the hype, either.