Car fuel mileage is not affected by a higher octane level. Many believe that the higher the octane level has a direct benefit on your fuel economy. The best gasoline for your car is stated in your owner's manual of the car. Most cars run optimally on regular gasoline. Luxury cars with higher quality and turbo engines may state premium is better. However, this is not because it will increase your gas mileage. People may be confused on what octane is and what it really means. The octane rating of your fuel is just how much energy that is needed to actually ignite the fuel. It has nothing to do with how much energy that the fuel actually puts out. These two concepts are different, and is the underlying reason as to why a higher octane level will not increase your fuel mileage. Not only will the octane level not effect your gas mileage, but it does not even increase the amount of power your car puts out. Again, this is because octane ratings are just how much energy is needed to ignite the fuel not how much energy is provided. There are only a few exceptions to the rule where high octane gas can actually help your fuel mileage. If your car specifically needs the high octane fuel for performance, which are rarer cases, but they are designed to run optimally on higher octane fuels. Without it, you will not get the best MPG ratings for your specific car.