Using The Right Fuel In Your Subaru And Which Grades To Avoid

Less Is Better

Should you run the highest octane fuel in your new Subaru? Won't it run better? The answer to both questions is only if it's required. The more expensive, higher 92 or 93-octane unleaded gas doesn't make your Subaru perform better unless you own a Subaru WRX or BRZ.

Which Fuel Is Best?

If you drive a Forester, Outback, Crosstrek, Ascent, Impreza, or Legacy, your vehicle is designed to run on 87-octane or regular fuel. It's the best grade for most Subaru models. If you drive a Subaru Solterra, you can disregard this report. See what I did there?

Some of the information for this report comes from Craig Fitzgerald, an automotive writer at SubaruDrive. 

Here is a complete fuel-type guide and what's best for each Subaru model: regular (87), mid-grade (89-90), and premium (91-92). Also, why does Colorado and other mountain states offer (85) octane, and should you use it?

Grades Of Gas

Gas stations in the U.S. typically offer three or more choices of fuel grades that vary by state. Colorado and some mountain states offer 85-octane gasoline. U.S. regulations require that each fuel grade be rated by its octane number and displayed on the pump. 

The most widely available fuel grades and their octanes are regular (87), mid-grade (89-90), premium (91-92), and, in some states, super-premium (93-94).

All new Subaru vehicles use a sophisticated four-cylinder Boxer engine (turbocharged and non-turbo engines). This unique engine design, also known as a flat four engine, features horizontally opposed pistons that move side-to-side, creating a lower center of gravity and better balance for the vehicle.  

Subaru Drive magazine says, "Every Subaru vehicle built in the past 30 years has relied on an increasingly sophisticated engine management system that constantly monitors the state of combustion inside the cylinders. Knock sensors relay information to the engine management system, and that powerful computer can automatically adjust the ignition timing to make the best use of the fuel in the tank."

What's The Best?

If you drive a Forester, Outback, Crosstrek, Ascent, Impreza, or Legacy, you don't need the more expensive grade of gasoline. Your vehicle is designed to run on 87 octane or regular gasoline, and you don't need expensive premium fuel. So, the best fuel for your Subaru is the one that meets these requirements and is readily available in your area.

Why does Colorado offer 85-octane gasoline?

87 is the lowest octane rating in most U.S. states and is considered "regular" gasoline. In Colorado and surrounding states, including Utah and Wyoming, 85 octane gasoline is "regular," and 87 is considered "mid-grade."

CPR News reports, "The reason for this difference comes down to altitude, said Mahesh Albuquerque, director of Colorado's Division of Oil and Public Safety at the Department of Labor and Employment. Air is less dense at higher elevations, reducing the chance of engine knocking when using a lower-octane gas."

The report says a 2015 study from the Coordinating Research Council, a nonprofit supported by the petroleum and automotive industries, showed using 85-octane gasoline at 5,000 feet was more fuel-efficient than using 87-octane gasoline at sea level. "Arguably, fuel economy with 85 octane at elevation was better than 87 octane at sea level," Albuquerque said.

Let's get technical

DriveSubaru magazine says, "In years past, the fuel-air mixture could ignite just by the force of being compressed rather than when the spark was precisely timed to ignite it. This damaging phenomenon is known as "preignition," and it was characterized by a knocking or pinging sound from the engine as it ran.

"To reduce this preignition, fuel producers began to measure their fuels against a standard known as isooctane, one of a family of hydrocarbons typically found in gasoline."

"A fuel's octane rating has nothing to do with the energy content of the fuel. It only shows how likely that fuel is to preignite."

Subaru's Performance Models

In the report, John Gray, Field Quality Assurance Manager with Subaru of America's Service and Quality Department, says performance vehicles like the WRX, WRX STI, and BRZ are designed to run correctly even when the optimal fuel grade isn't available.

The Owner's Manual states for WRX and BRZ models: "The engine is designed to operate using unleaded gasoline with an octane rating of 91 or higher." However, fuel with an octane rating of 87 or higher can be used in either model without damaging engine durability.

You may notice a small drop in performance using lower-octane fuel, but the vehicle's engine management system will automatically adjust engine timing, avoiding a preignition condition. This means that while your vehicle may not perform at its peak, it will still run smoothly and efficiently without any risk of engine damage.

What About The WRX STI?

Subaru says the higher-performance Subaru WRX STI requires 93-octane gasoline for optimal performance. The Owner's Manual states that if 93 octane isn't available in your area, 91 octane is acceptable. The WRX, WRX STI, and BRZ also have decals inside the fuel lid, defining the fuel requirements.

Less Is More Money In Your Pocket

Except for the Subaru WRX, WRX STI, and BRZ sports cars, the Forester, Outback, Crosstrek, Ascent, Impreza, and Legacy models are designed to run on 87 octane. You don't have to buy the more expensive grades. 

In the higher-elevation states of Colorado, Wyoming, and Utah, you can buy 85-octane gasoline. In other areas of the country, your Subaru model is designed to run on 87 octane. Buying regular 87-octane gasoline and avoiding premium gas can save you hundreds of dollars in additional fuel costs annually. It's a smart way to manage your expenses without compromising on your Subaru's performance.

I am Denis Flierl, a top Torque News reporter since 2012. I’ve invested over 13 years in the automotive industry in a consulting role, working with every major car brand. I am an experienced Rocky Mountain Automotive Press member. You'll find my expert Subaru analysis here. Follow me on my X SubaruReport, All Subaru, WRXSTI, @DenisFlierl, Facebook, and Instagram.

Photo credit: Subaru