Short Answer: If your car says premium is “Required” then you should use premium in your car.  If it says premium is “Recommended” you probably should save the money and skip it.  Below is an explanation of what premium gas is, why it matters and then links to lists of cars where premium is merely recommended and where it is required.

Longer Answer: First, a quick primer on how a four-stroke internal combustion engine works (this is the type of engine you’ll find in your car unless you drive an electric car or a Mazda with a rotary engine). An internal combustion engine works by creating explosions by igniting gasoline in the engine’s cylinders. These explosions push down pistons which in turn move a crankshaft. This process works in four strokes as follows:

1. Down stroke one: the piston moves down inside the cylinder allowing the cylinder to be filled with a mixture of air and gas.
2. Up stroke one: the piston moves up and compresses the air and gas which makes the mixture more volatile and explosive..
3. Down stroke two: at the highest point when the piston has fully compressed the air and gas the spark plug ignites the gas and air mixture.
4. Up stroke two: the piston moves up, expels the remains of the burnt fuel and air out the exhaust and then the process starts all over again.

Higher performance engines are designed to operate with higher compression. With lower octane gasoline, sometimes the compressed gas/air explodes prior to reaching the top of the up stroke and prior to spark plug ignition.  This occurs because of the high compression of the more volatile lower octane gasoline can  sometimes creates ignition without a spark. These premature ignitions create knocking sounds which can damage the engine and result in lower gas mileage and performance.  Thus, if your car requires premium gas, you should use premium gas. Failure to do so can cause knocking or decrease engine performance and gas mileage.

However, many cars merely state that premium gas is recommended rather than required.  In that case you are likely throwing your money away. Check out this article from Consumer Reports which tested cars for gas mileage and acceleration that merely recommended premium gas and they found no difference between premium and regular: http://www.consumerreports.org/fuel-economy/why-you-might-not-actually-need-premium-gas/

Link from Edmunds for 2011-2016 cars that merely recommend premium: https://static.ed.edmunds-media.com/unversioned/img/pdf/premium.gasoline/premium.fuel.recommended.031816.pdf

And here is the list of required premium fuel models: https://static.ed.edmunds-media.com/unversioned/img/pdf/premium.gasoline/premium.fuel.required.031816.pdf

Still reading and curious? Here’s a great explanation from Scientific American about what an octane rating is: “All gasoline is a heady brew of many different hydrocarbon molecules, ranging from heptane (seven carbon atoms and 16 hydrogens)  to decane (10 carbons and 22 hydrogens) and beyond. The hydrocarbon clearly identified on the pump is octane (eight carbon atoms and 18 hydrogens). This number, however, is not a measure of the percentage of octane actually in the gas itself. Rather, it is a measure of how that gasoline compares with a pure mixture of octane and heptane. At special laboratories across the globe, chemists concoct such reference fuels and then use them in comparison with refined gasoline following the dictates of standardized measures.” Octane is less volatile than heptane.

FINALLY – if you are concerned about hurting your car that recommends premium if you use a lower octane gas – you are probably ok given that modern engines have control systems that adjust the timing of spark plug ignition to account for lower octane fuels and prevent knocking.

Whew.  Long IFOD.