You may have heard that using higher octane fuel than your vehicle is rated for will raise fuel mileage and save you gas. The rumor floating out there also says that the money you save in fuel economy will be greater than the extra cost for the premium gas.
Even when searching for data to support this theory nothing can be found. It is mostly anecdotal and word of mouth. Octane rating is a measure of the fuel’s resistance to premature detonation. This means that a higher octane fuel like 93 compared to 87 takes a higher amount of energy to initiate the combustion reaction within your vehicle’s cylinders. What this means is a lower octane fuel has a higher propensity for detonating prematurely and causing the famous engine knock.
However a vehicle designed to run on 87 octane, as most vehicles are today, will not benefit from a higher octane fuel. It will just waste money. However, if your vehicle is designed to run on premium fuel including many sports cars you can lose fuel efficiency by switching to a lower rated fuel. The rule of thumb here is to use nothing lower than what your vehicle’s owner manual calls out for. If your car was designed to run 87 octane use 87 octane. If the owners manual calls for 89, use 89.