If you ask average drivers the difference between premium and regular gas, chances are they won’t be able to say much beyond “higher octane” — a well-advertised but nebulous concept that no one really can explain.

At least, such was the case around the Yahoo Finance newsroom.

However, that lack of understanding hasn’t stopped 16.5 million US drivers from spending $2.1 billion last year on “premium” gas car manufacturers did not require, according to a report AAA released this week.

With a loaded name like “premium,” it’s not hard to see why people associate high-octane gas with quality and choose to treat their cars to top-shelf gas in search of better performance or longevity. But after a 12-month comprehensive study to see if high-octane gas benefited regular engines, AAA found absolutely no benefit in performance, fuel economy or emissions.

As John Nielsen, AAA’s managing director of automotive engineering and retail, succinctly put it, “Premium gasoline is higher octane, not higher quality.”

That’s not to say it isn’t important to use in some cases. In high-performance engines, the gas-air mixture gets massively compressed by the piston, and the higher octane prevents the fuel from igniting too early, when the piston isn’t quite in the right place. When this happens, a frequent “ping” or a “knock” sound can occur.

But feeding premium high-octane fuel to a regular engine that doesn’t have that massive compression of the gas-air mixture is like buying gluten-free bread when you don’t have celiac disease. You may feel like you’re doing your car a favor, but it doesn’t notice or care.

AAA may counsel you against wasting your money on premium fuel for your regular car, but the automotive information site Edmunds takes it a step further and notes it’s possible your high-performance car may not even require the hydrocarbon cocktail. It might simply be recommended—which means you should feel comfortable experimenting with cheaper regular gas, according to Edmunds.

If you’re a premium fuel user, you can see whether you can experiment by checking your manuals for specific guidance. Or if your car is between the model years of 2011 and 2016, you can just look for it on Edmunds’s “premium recommended” list and its “premium required” lists.

So how much does all this matter really? Well if you drove 12,000 miles last year with a car that gets 25 miles per gallon, saving $0.40 on every gallon would have added up to a almost $200.

Ethan Wolff-Mann is a writer at Yahoo Finance focusing on consumerism, tech, and personal finance. Follow him on Twitter @ewolffmann.

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