Latest Reader Question: Octane Recommendation (April 24, 2018)

Here’s the latest reader question, along with my reply!

Jim asks: Hi Eric, Just read your article on octanes. I drive a ’06 Mazda RX-8. Where do you think that engine would fit? Better to use lower or higher octane fuel?

My reply: Don’t ask me – ask Mazda! Octane recommendations should be listed in your owner’s manual or on a sticker located on the inside of the fuel door. It will generally say one of three things:

Unleaded fuel only – This means you can (and should) use regular gas to get optimum mileage and power out of the engine.

Premium only – This means you should only use premium (high octane) fuel. Use of lower octane fuel will not hurt the engine; it’s equipped with knock sensors and can automatically adjust to accommodate lower octane fuel. But you will probably notice both a mileage and fuel economy downtick.

Premium fuel recommended – This one’s confusing because it amounts to the same thing as premium fuel only. It means the engine will deliver its best performance/mileage if fed high-octane premium fuel. But it publicly concedes that it’s safe – no damage will result to the engine – if you use regular. You’ll just notice a downtick in mileage/performance, which may be worth it to you given the much lower cost of regular fuel.

There is a fourth variable as well.

Mid-grade fuel. It has a higher octane rating than regular – but not as high an octane rating as premium. It’s generally 89 octane or so-  vs. 90-something for premium (and 87 or so for regular).

Use of mid-grade may actually give you best mileage/power – depending on your particular car’s compression ratio, etc. This varies considerably from car to car. “High compression” generally (historically) meant any engine that had a CR of around 9.5:1 or higher. But some current/recent car engines have compression ratios of 11.0:1 or even higher (e.g., Mazda’s SkyActive line of engines). These latter probably do best on the highest available octane, while an engine with 10.0:1 CR might actually do better on mid-grade.

Keep in mind, also, that the octane rating of fuels – all three grades – available today are probably not the same as what they were when your car was made. Today’s mid-grade, for example, is close to the same (and even possibly higher) octane than premium was back in the ’80s.

You can experiment to find out what works best in your car, starting with the grade that best matches the original factory recommendations.

Fill up the tank with premium – if that’s what Mazda recommended. Take note of the mileage – and, if you can measure it objectively-  the car’s performance. Then refill with mid-grade and note the differences, if any.

PS: Another variable to take into account is additive packages. “Premium” fuel may have more additives than “regular.” Mid-grade might have the same additive/detergent package as “premium,” but  costs less per gallon – saving you money if you don’t need the higher octane.

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Got a question about cars – or anything else? Click on the “ask Eric” link and send ’em in!