If you’ve ever owned a luxury/performance car (or any vehicle that recommends 91+ octane gas for that matter), you’ve more than likely asked yourself the question, Is it OK to fill my car up with regular (87 octane) gas? Am I going to damage my engine?

You may have heard that if you use regular fuel in a car that requires premium, your engine will begin knocking (that is, the fuel detonates prematurely inside the engine’s cylinders). Others will claim that in any modern car, the ECU will work with your engine’s knock censors to maximize performance for whatever fuel type you’ve put in your tank. Fortunately, that debate is not the purpose of this post.

What I wanted to determine was whether or not using regular fuel in a car that requires premium is even financially advantageous. That is, does using regular fuel take a toll on your gas mileage, and if so, is this difference enough to offset the ~$0.25 that you’re saving on each gallon of gas?

You might say that the definitive way to test this would be to fill up a car using 91 octane for a few weeks, calculate the MPG, and then do the same using 87 octane. Unfortunately, these results wouldn’t bode well as an overall conclusion that could be recommended to anyone, since your Acura TL’s V6 probably isn’t going to respond the same way the W12 in your neighbor’s Audi A8 will. It’s also likely the results will vary depending on whether the engine is naturally aspirated or turbo/supercharged. Not to mention the effects your particular driving style or your split between highway/city miles.

So, I decided to do some googling for people who had independently performed this sort of experiment in their own cars and calculate the overall average of these values. It wasn’t as easy as I thought; there were plenty of people stating things like “I didn't notice much of a difference when using regular gas,” however it was somewhat difficult to find people who had [claimed to] perform the experiment seriously, tabulating their mileage over multiple fill ups for each octane and recording their MPG to at least one decimal place.

Here’s what I found:

Over the 8 cars, we see that on average, gas mileage took a 4.5% hit when regular fuel was used. However, it’s important to note that two of the cars recorded worse gas mileage using premium fuel [go figure]. Overall, the average cost per mile comes out to $0.167 using regular versus $0.172 using premium. In other words, for every 1,000 miles driven, regular gas yielded an average savings of just $5.31.

So, what’s the conclusion? If you want to get an accurate answer, there’s no substitute for performing the experiment in your own car, according to your own driving style. If you’re too lazy to do that, you’re going to have to make do with what we’ve calculated here. Is a $5 or even $10 savings for every 1,000 miles you drive really worth the reduced performance and potential for long term damage to your engine?...You be the judge.

Of course, with the money you'll save using AutoGlance, you mite not even have to worry about any of this...

Reddit comments on this article here.

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