Fill ‘er up. At the pump, is premium or regular best for your ride?

Choosing between regular and premium gasoline is one of the most common — and most misunderstood — decisions drivers make. The confusion is expensive, because “premium” sounds like an upgrade even when it isn’t.

Here’s the simple rule that saves real money: buy the fuel your engine is designed to use. If the owner’s manual says regular is fine, premium won’t give you extra horsepower, extra mileage or extra engine life. It will mostly give you a bigger receipt.

And that receipt can add up quickly. According to AAA, the average price of regular unleaded in Georgia was about $2.66 per gallon on Monday, compared to $3.53 for premium — a gap of 87 cents. Fill a 15-gallon tank and you’re paying roughly $13 more for premium. Do that weekly, and you’re well into “new tires” money over a year.

Car shopping tip: Before you buy, check whether the engine needs premium. Over a few years, that fuel bill can materially change the cost of owning the vehicle.

What ‘premium’ actually means

Premium gasoline is mostly about octane, the number on the yellow pump sticker. In much of the country, regular is 87 and premium is typically 91 to 94.

Chris Hardesty is a veteran news researcher and editor who provides advice on buying, owning and selling cars for Kelley Blue Book and Autotrader. (Courtesy of Cox Automotive)

Octane is not a measure of how “strong” the gas is. It is a measure of the fuel’s resistance to knocking, also called pinging or detonation. Knock happens when the air-fuel mixture ignites at the wrong time inside the cylinder. Mild, occasional knocking is not always catastrophic, but sustained knocking can damage an engine.

Why some engines need it

Many modern engines squeeze more power from less fuel by raising pressure and heat in the combustion chamber. Turbochargers and high compression ratios are two common ways to do that. Higher octane fuel is better at handling those conditions without knocking, which is why premium is often specified for performance cars and some turbocharged family vehicles, too.

But “often” is not “always.” Plenty of turbocharged engines are designed to run on regular, and some luxury models are tuned to work on regular even if they deliver their best numbers on premium. There is no reliable rule of thumb based on brand, price or whether the vehicle is “sporty.”

Required vs. recommended

What matters is the language in your owner’s manual and any label inside the fuel door. Automakers might say premium is “required” or “recommended.”

Required means the engine was designed around higher octane. The vehicle’s computer can sometimes protect the engine if you use regular in a pinch, but it will usually reduce spark timing and boost to prevent knock. That protection can mean slower acceleration and lower fuel economy, and it is not a good long-term plan.

Recommended means the engine can run on regular, but premium might deliver full advertised performance — especially in hot weather, at high altitude, while towing or when you demand hard acceleration. If you don’t notice a difference, your wallet has the best vote.

If your vehicle is designed for 87, premium generally doesn’t provide a measurable benefit.

What happens if you use the ‘wrong’ fuel?

• If your car is designed for regular and you buy premium:

Nothing bad happens mechanically. You simply spend more money for little to no return.

• If your car requires premium and you buy regular once:

In most cases, you won’t immediately damage the engine. You might notice sluggish acceleration, and the engine may pull timing to prevent knock.

• If you do it regularly in an engine that requires premium:

That’s where the risk rises. Persistent knock can mean hotter operation, more stress and potentially expensive issues over time. The real-world impact varies by engine design and driving conditions, but it’s not a smart place to gamble.

Can you mix grades?

Topping off with a different grade is generally OK. If you mix half a tank of 87 with half a tank of 93, you’ve effectively created something in between.

What mixing won’t do is create a “best of both worlds” bargain if your car only needs regular. The engine can’t “unlock” extra benefits from octane it doesn’t need.

What about ethanol?

Ethanol content is separate from octane.