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This is one of those car topics that gets debated like politics or religion. This article isn’t meant to fan those flames but instead it’s to share what I’ve observed so you can make your own decisions.

Most modern factory-produced cars including European cars like BMW have a knock sensor which can help adjust the car’s timing based on ignition behavior. So if you have a car that is supposed to be fed premium 93-octane fuel is instead fed regular 87-octane then the engine experiences what they call early detonation (ie. engine “knock”). The knock sensor senses that and adjusts the car ignition timing to reduce/eliminate early detonation and the car seems to behave normally (unless you notice some small decline in power/performance).

The reverse doesn’t apply though. If you fed premium 93-octane to a car that is supposed to be fed regular 87-octane then you may not experience any noticeable change. If you had a classic, historic car with high mileage then it might help reduce engine knock but that’s about it. If you’re just doing it anyway for some reason then I don’t want to give you the bad news: feel free to Google the reasons why you shouldn’t.

In the past I’ve temporarily switched octanes with previous cars but thought I should try doing it longer and report back what I observe. For the past 10 months I’ve been feeding regular 87-octane gas to a 2004 Saab 9-3 Arc that should be getting 93-octane. I got the car with 116k miles on the odometer and have put 14k miles to-date so I think it’s a good time to provide my observations:

Easier on the wallet:

Unlike my BMW M3 which I religiously feed 93-octane, feeding the Saab 9-3 regular 87-octane just “feels” affordable and “happier” when I’m at the pump. I can’t say that the M3 and the 9-3 share similar MPG specs (they don’t) but just saving at least $0.20 / gal over the 14k miles I’ve driven to-date calculates to about $93 saved. Cha-ching!

Not quite as powerful:

As I get older and more “routined” I don’t have a need or desire to drive at NASCAR levels each day I head to work. So losing a little horsepower to save some coin at the pump with a car rated for premium gas is OK. It’s not like it drives as slow as a bus, it just feels slightly less powerful. It’s an almost negligible feeling for me as I can still have spirited-enough drives with it.

Car runs “fine”:

The car doesn’t sound weird or act different than what I’d expect. My prioritization's for a car is largely based on its objective: the BMW M3 is what I consider a “fun” weekend car whereas the Saab 9-3 is a daily beater/commuter. Both are rated for premium 93-octane but I think each is “fine” based on what I expect from it whenever it’s driven. So for me the BMW M3 is “fine” if it always running at it’s rated power levels and the Saab 9-3 is “fine” if it comfortably gets me from point A to point B.

I stuck to quality gas:

One thing I didn’t want to experiment with is feeding the car the cheapest gas possible so I’ve always fed it with Top Tier gasoline. The last thing I wanted was some shoddy quality gas screwing up my engine somehow. Top Tier brands have a standard where gas quality has been certified all the way to the pump of each gas station. Brands that aren’t Top Tier can’t guarantee that the formulated quality at the manufacturing plant will meet the same standard at the pump of every gas station. I’ve been sticking to BP gas which is Top Tier brand but there are others you can choose from.

So far I’ve been satisfied with my test on the Saab 9-3 and will continue to feed it regular 87-octane gasoline. You can experiment with your car if you wish, unless you intend to keep your beast running in “fun” mode full-time.

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Written by :
Gerry Tseng

Gerry lives in Cincinnati, OH where he works as a data analyst by day and enjoys working on cars in his free time. He’s spent over 25 years on domestics + imports alike. His latest efforts include a Saab 9-3, a BMW M3 and a Volvo S60.

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