Regular or Premium Gas?

Use the grade your owner's manual calls for. If it says regular, 87 octane is what the engine was designed and tested on, and paying for premium buys nothing measurable. If it says premium required, use premium. The rest of this article explains the cases in between, and the labels at the pump that matter more than most drivers realize.

The fuel section of the manual is usually near the back, and many cars repeat the requirement on a label inside the fuel door.

What octane measures

Octane is a measure of knock resistance, nothing else. Knock, sometimes heard as pinging, happens when part of the fuel and air mixture ignites on its own from heat and pressure instead of burning smoothly from the spark plug. Engines with high compression or a turbocharger squeeze the mixture harder, so they need fuel that resists igniting early.

The number on the yellow pump label in the United States is the Anti-Knock Index, the average of two lab ratings called the Research and Motor octane numbers, which is why the label often reads "(R+M)/2 method." Regular is typically 87, midgrade 89 or 90, and premium 91 to 94.

A higher number does not mean more energy. Premium does not hold more power per gallon than regular, so it does not raise horsepower or mileage in an engine tuned for 87. Midgrade at many stations is simply blended at the pump from the regular and premium tanks, and few cars ask for it.

"Premium required" versus "premium recommended"

Modern engines have knock sensors, small microphones bolted to the block that listen for knock. When they detect it, the engine computer retards the ignition timing until it stops. That protection is why the two phrases mean different things.

Premium recommended means the engine is tuned to produce its rated power on premium but is designed to run safely on regular. On 87, the computer pulls timing back, and you may notice slightly less power and sometimes a small drop in mileage. Whether the savings are worth it is your call, and nothing is harmed either way.

Premium required means the engine was not designed to rely on that protection all the time. A single tank of regular in an emergency is usually survivable if you drive gently and avoid heavy throttle, but some manuals warn that repeated use can damage the engine and may not be covered by the warranty. Persistent knocking on the correct fuel is a reason to see a mechanic.

Altitude and 85 octane

Drivers in parts of the Mountain West, including Colorado, Wyoming and Montana, will see 85 octane sold as regular. Thinner air at altitude lowers cylinder pressure, which reduces the tendency to knock, and older carbureted engines without electronic controls could get by on lower octane up there.

That logic does not carry over to modern engines. Fuel-injected engines with knock sensors and turbochargers adjust for air density on their own, and many manufacturers specify 87 as the minimum regardless of elevation. The safe rule at altitude is the same as at sea level: buy the grade in your manual, which usually means skipping the 85 button and choosing the next one up. Altitude is not a reason to switch a regular-fuel car to premium either.

Top Tier detergent gasoline

All gasoline sold in the US must contain a minimum level of detergent set by the EPA. Top Tier is a voluntary standard, launched in 2004 by a group of automakers including GM, Honda and Toyota, that requires more detergent than that minimum to limit carbon buildup on fuel injectors and intake valves. A 2016 AAA study found that the non-Top Tier gasolines it tested left considerably more engine deposits than Top Tier brands.

Top Tier applies to every grade a licensed station sells, so regular from a Top Tier station meets the same detergent standard as its premium. The program's website lists participating brands. Detergent level matters for all engines; octane only matters for the ones that call for it.

Ethanol blends: E10, E15 and E85

Most regular gasoline in the US is E10, up to 10 percent ethanol, and every modern car is built for it. Ethanol carries less energy than gasoline, so mileage on E10 runs roughly 3 percent lower than on pure gasoline.

E15, sometimes sold as "Unleaded 88," is approved by the EPA for cars and light trucks from model year 2001 onward. It is not approved for motorcycles, boats, lawn equipment or older vehicles, and a few manufacturers' manuals still advise against it, so check before using it.

E85 contains between 51 and 83 percent ethanol depending on season and region, and it is only for flex-fuel vehicles. Those are usually marked with a yellow fuel cap or ring, a badge on the body, or a note in the manual. Putting E85 in a car that is not flex-fuel can trigger a check engine light and cause drivability problems, and because it holds less energy per gallon, flex-fuel owners should compare cost per mile rather than the price on the sign.