While octane booster fuel additives can produce measurable gains of octane in gasoline, these gains are unlikely to improve fuel economy, as octane ratings have little influence on fuel consumption. Some cars achieve worse mileage on high-octane gas, so it is possible these products may actually reduce fuel efficiency.
Octane ratings for gasoline are measurements of the relative purity of the hydrocarbon mixture in the fuel in terms of combustion efficiency, but do not actually measure the amount of octane in the fuel itself. This is one of the reasons that some racing and airplane fuels have a rating above 100, as they burn more efficiently than a fuel composed of pure octane does. While a higher efficiency rating suggests that higher-octane fuels should contain more energy per gallon and therefore help a car obtain higher fuel mileage, in practice the differences are negligible. However, higher-octane fuels can sometimes produce measurable increases in the power output of an engine, so octane boosters can be useful for this purpose.
The major reason to use high-octane fuel blends in older engines was to reduce knocking, a phenomenon that occurred when impurities in fuels with low octane ratings caused premature detonation of the fuel-air mixture in the engine cylinder. However, modern engines use computer-controlled ignition systems that virtually eliminate knocking regardless of the octane rating of the fuel used.