Among drivers whose car owner’s manual recommends regular fuel, about 5 percent use mid-grade and 3 percent use premium, according to a recent, nationally representative study by the Consumer Reports National Research Center. Based on 12,000 miles of driving per year, average nationwide fuel prices in September, and fuel economy of about 24 mpg, people who use mid-grade fuel unnecessarily fork over an extra $50 per year for gas. Those who use premium unnecessarily pay an extra $120. Translated nationally, that’s an unnecessary added cost of roughly $1.5 billion.
Octane grades don’t represent "good, better, best"; they measure a fuel’s resistance to preignition (pinging or knocking), a condition in which gasoline burns uncontrollably in the engine’s combustion chambers.
Even many cars for which premium gas is recommended can use regular, because most modern engines can detect preignition and automatically make adjustments to eliminate it.
Check your owner’s manual or fuel-filler door. If it says to use regular fuel, do so—there’s no advantage to a higher grade. If it says "premium required," use premium. If it says "premium recommended," try regular. Keep a log of your fuel economy. If you don’t see a change, keep using regular and enjoy the savings.