Most fuel additives do little to improve the performance of a vehicle, according to How Stuff Works. They cannot make the engine more powerful or improve fuel economy.
An additive that increases octane may improve performance if using a lower than recommended grade; however, many of these additives make no measurable difference in the octane, and quality fuel is better than an additive. Fuel stabilizers are useful in engines that sit for long periods and for the fuel in small engines, such as lawnmowers at the end of the mowing season. Fuel system cleaners offer little benefit to the vehicle owner. If the vehicle is experiencing a problem due to mechanical failure, fixing the underlying problem is a better option.