If you're putting premium gasoline in a car that does not require it, you're wasting money.

Premium gas is not purer or cleaner than its alternatives, but it is a lot more expensive — typically by about 30 cents per gallon.

The difference is that premium gas is less combustible. It has a higher percentage of octane (for example, 89%, 90%, or 91%) which is resistant to combustion, and less heptane, which is much more likely to ignite when compressed, according to How Stuff Works.

The more fuel can be compressed before it ignites, the more powerful an engine can be.

Does your non-fancy car need fancy gas?

No. According to the Federal Trade Commission (FTC), charged with consumer protection, there’s no benefit to putting premium gas in the tank of a car that does not need it.

The EPA requires that all gasoline contain engine cleaning detergent additives, so premium isn’t better than regular at preventing the buildup of engine deposits.

“In most cases," the FTC says, "using a higher octane gasoline than your owner's manual recommends offers absolutely no benefit.”

The exception is if you hear your car “knocking.” The rattling sound is triggered when gasoline combusts before it’s ignited by the spark plug, which disrupts the proper working of the engine. Per How Stuff Works, pre-ignited fuel does not burn completely, and leaves behind debris that makes things worse over time.

In that case, higher octane fuel may help, because it is less likely to pre-ignite. But, the FTC notes, the better long-term solution is to have your engine repaired, then go back to regular fuel.

Does you fancy car need fancy gas?

If your owner’s manual says to use premium gas, you should listen. That’s most likely if you drive a luxury or sports car.

High-performance engines tend to run hotter and use a high compression ratio, so fuel that is less likely to pre-ignite keeps things running smoothly.

According to Consumer Reports, you can use regular gasoline with engines that call for premium, if you're ready to sacrifice some performance. That’s because many cars that need premium also have built-in knock sensors, which detect pre-ignition and adjust the engine’s timing to avoid it.

The result is a bit less power and worse fuel efficiency. So if you're willing to make the sacrifice, and the lower fuel economy doesn't outweigh what you save at the pump, think about dropping down a grade.