If you're still in doubt about switching to a lower-octane fuel, here's a more in-depth explanation of why the change is unlikely to hurt your car:
First of all, premium gas has a higher octane rating, an important factor in helping prevent engine knock or "pinging." Depending on where you live, this premium-grade fuel could be 90 octane, 91 octane or even 94 octane.
That's one reason premium costs more. To increase the octane rating and reduce knock, refineries use a more elaborate process that blends an expensive substance called alkylate into the fuel.
What is engine knock? After vaporized fuel mixes with air and fills the combustion chamber, rising pistons compress it. The spark plug then fires, initiating the combustion process. But combustion doesn't happen instantaneously. It takes some time for the flame kernel to develop, grow and then burn the entire contents of the fuel-air mixture in the cylinders. There's a critical period in the middle of this process when the mixture in the unburned region is being heated by the neighboring burned gas while being compressed by the rising piston.
This one-two punch can cause the mixture in the unburned region to self-combust rapidly in an uncontrolled fashion. That's the knock, which hurts power and can damage the engine. Simply put, high-octane gasoline can be compressed and heated to a greater degree without self-igniting. That's why high-performance engines use premium fuel.
In the old days, engines could not deal with fuels of varying octane ratings. Using the wrong fuel would make the engine knock and possibly damage internal engine components. But today, engine control systems can compensate for low octane by adjusting ignition timing to avoid knocking. This sophisticated electronic capability effectively tunes engines on the fly and gives drivers more flexibility in the grade of fuel that they can use safely.
Compared to premium gas, lower-octane fuels don't allow the engine to run as much ignition advance during situations calling for rapid acceleration. More ignition advance causes spark plugs to fire sooner, which (within limits) allows the engine to make more power — and accelerate more quickly — under these conditions. Since the engine doesn't make quite as much power with low-octane fuels, this translates into slower acceleration in cars for which premium fuel is recommended. The performance loss is especially noticeable in turbocharged gasoline engines, which have become increasingly popular in recent years.
The performance loss, however, is something you will only notice if you have a heavy foot and accelerate rapidly from a dead stop or when you change lanes at highway speeds. But if you accelerate moderately, the loss of power may not be noticeable whether you use premium or regular-grade fuel.
When premium can be a money-saver
Edmunds has noted, however, at least one case in which a car with a small turbocharged engine returned better fuel economy when running on premium. The owner's manual of the car in question, a 2011 Chevrolet Cruze LTZ, called for regular unleaded gasoline. Yet in a specific test, we noticed that we got better fuel economy, and ultimately saved a bit of money, using premium fuel. We conducted the testing in the scorching Death Valley, which might have influenced the outcome.
Since then, small turbocharged engines have become increasingly common in new cars. In 2008, three years before our test, turbocharged engines were standard on only 10% of vehicles. These days, it's surprising when a car doesn't have a turbocharged engine.
There's no guarantee that all of the overachieving engines will benefit from higher octane as the Cruze did in our 2011 test. You can try this yourself to see if you can save money by using premium fuel.
Monitor your fuel economy and performance over at least two tanks of premium fuel. Record the trip mileage, gallons used, fuel price and octane rating in a notebook or with an app such as Fuelly. If your car has an onboard fuel economy meter, make sure you reset it when filling up. Then, fill up on the same number of tanks of regular gasoline and record all the same data. Finally, compare the results. You're looking for a drop-off in fuel economy or a sense that the car is slower or hesitant under strong acceleration. That's the drill for a premium-recommended car. You can stay with premium or step down to regular unleaded if you prefer.
It's a different story for a car whose engine requires premium fuel. The car will run on regular fuel in a pinch, but you shouldn't make a habit of it. The fuel's lower octane can result in elevated exhaust-gas temperatures and possible knocking, both of which can adversely affect the engine's health in the long run. Running regular-grade fuel in a car that requires premium might sound like an easy way to shave a car's operating costs, but the short-term savings won't come close to offsetting the cost of repairs to a damaged engine.
For those driving premium-recommended cars, however, it's just a matter of driving moderately and avoiding acceleration with a wide-open throttle. Do that and you might never feel the difference between using premium and regular-grade gasoline. And neither will your car.