Are there fuel additives that will increase my gas mileage?
Well, I won't make that an absolute no. It depends on how old your car is. If you own a car that was built in the last 15 years or so, it has programmed fuel injection. A computer monitors a number of sensors located at various points in your intake system, the engine itself, and the exhaust. The computer knows when the engine is cold, and delivers the extra fuel it needs to warm up. When the engine is at operating temperature, it monitors the amount of oxygen in the exhaust gas to adjust the air/fuel ratio to its ideal proportions. This is called the stoichiometric ratio. For a gasoline engine, this is 14.7 parts of air for each part of fuel, by weight, and the computer adjusts fuel delivery to maintain this ratio. No fuel additive will have any real effect on it. The stoichiometric ratio for different types of fuels varies, but when you consider how these things are sold - in 8-oz bottles, for example, anything you'd add like this would have a negligible effect on the composition of what's in a 12 to 20 gallon tank. This being said, there are some fuel additives that can help to clean injectors that have gotten fouled with age. This will make them work better, and might save you some gas. However, if your injectors are fouled, it's probably time for a general tune-up. Finally - your gas mileage will vary with the kind of driving you do. Slow down, if you can, or take a different route to work, or leave at a different time, to avoid some of the stop and go traffic that kills gas mileage.
Everytime gas goes up or there is a shortage, charlatans come out of the woodwork with all sorts gadgets and gimmicks for increasing fuel mileage. When tested in the lab, none produce the claimed results and any that work at all don't increase it enough to justify the cost of the product. Your best bet is to keep the car tuned properly, keep the tires properly inflated and reduce the weight by cleaning out the junk in the trunk. Your only other options would be to drive less (by grouping errands, carpooling or use other forms of transportation) or to trade the car for something that gets higher mileage.
There is one fuel additive that decreases your fuel economy in cars that have fuel injection systems with closed loop oxygen sensors used to adjust fuel to air ratios on-the-fly. that is Ethanol alcohol. Where gasoline has an optimal burn rate at about 14.7:1 (parts gasoline to air by weight) Ethanol is about 19:1. So even a 10% mix of Ethanol reduces your fuel economy by about 5%. So if you can find a station selling pure gasoline you will notice your fuel economy go up just a bit. Running any alcohol in a car not designed for it is bad. they set the limit at 10% with the idea that the damage this causes to rubber seals, carburator floats, rubber or aluminum fuel lines will be so gradual that your car will be worn out before the damage becomes apparent. Otherwise. the only other thing that has promise is the Fitch Device. a catalyst that changes the nature of gasoline slightly has been proven on the dynometer to increase rear wheel horsepower slightly and fuel economy quite measureably. You can also go to a lower (and cheaper) octane rating in your car with this device.