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Impact of regular/87 octane gas on fuel economy?

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Impact of regular/87 octane gas on fuel economy?

 
Old 06-08-2019, 03:15 AM
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Default Impact of regular/87 octane gas on fuel economy?

Here in Central Florida regular gas costs 20-24% less than premium gas and since premium is recommended but not required by Audi I decided to try an experiment to see what the impact would be on fuel economy by switching to regular (87 octane) fuel for a little while. I tried to drive my 'normal' style of brisk acceleration and cruising at 76-84 MPH on the interstate (2/3 - 3/4 of my mileage) during this experiment.

After 4 full tanks of regular fuel my long term fuel economy went from 29.2 to 29.7 mpg and my average speed went from 37 to 38 mph. I was expecting the long term fuel economy to drop slightly so I was rather surprised that it actually went up.

Acceleration seemed to be very slightly slower with the regular gas as opposed to my normal 93 octane fuel.

I calculate it would save me about $500/yr in gas cost using regular.

Has anyone else tried running regular gas in their B9 A4?

PS: Still not sure how Audi calculates the estimated fuel range. It's clearly not using the short term fuel economy (9.4mpg) and 15.3 gal (fuel tank size) * 29.7mpg (long term fuel economy) = 454 miles (not close to the reported 535 miles).

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Old 06-08-2019, 03:55 AM
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Originally Posted by Onewolf42 View Post
Here in Central Florida regular gas costs 20-24% less than premium gas and since premium is recommended but not required by Audi I decided to try an experiment to see what the impact would be on fuel economy by switching to regular (87 octane) fuel for a little while. I tried to drive my 'normal' style of brisk acceleration and cruising at 76-84 MPH on the interstate (2/3 - 3/4 of my mileage) during this experiment.

After 4 full tanks of regular fuel my long term fuel economy went from 29.2 to 29.7 mpg and my average speed went from 37 to 38 mph. I was expecting the long term fuel economy to drop slightly so I was rather surprised that it actually went up.

Acceleration seemed to be very slightly slower with the regular gas as opposed to my normal 93 octane fuel.

I calculate it would save me about $500/yr in gas cost using regular.

Has anyone else tried running regular gas in their B9 A4?

PS: Still not sure how Audi calculates the estimated fuel range. It's clearly not using the short term fuel economy (9.4mpg) and 15.3 gal (fuel tank size) * 29.7mpg (long term fuel economy) = 454 miles (not close to the reported 535 miles).

Interesting. My way of saving a little involves buying a tank of 93 followed by a half tank of 87/89.
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Old 06-08-2019, 05:04 AM
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I wouldn't recommend using regular gas and neither does Audi. There could be some long term affects on your engine if it constantly hits your knock sensor, but it's your car. Just want to throw that out there.

Last edited by Nickodarius; 06-08-2019 at 05:07 AM.
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Old 06-08-2019, 05:51 AM
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Originally Posted by Nickodarius View Post
I wouldn't recommend using regular gas and neither does Audi. There could be some long term affects on your engine if it constantly hits your knock sensor, but it's your car. Just want to throw that out there.
Here's the 2018 A4 owner's manual page 310 about gasoline. It seems to indicate that it's ok to use 87 octane if premium/91 octane is not available?


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Old 06-08-2019, 08:29 AM
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Iíve known so many lease owners using regular on premium only vehicles. No short term problems but thatís why I refuse to get a CPO unless itís a dealer car or something.
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Old 06-08-2019, 08:36 AM
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Originally Posted by JDA_yeg View Post
Iíve known so many lease owners using regular on premium only vehicles. No short term problems but thatís why I refuse to get a CPO unless itís a dealer car or something.
I am on the same page with this.
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Old 06-08-2019, 09:05 AM
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From what I've researched, octane-increasing ingredients actually have (marginally) lower energy content than the base gasoline with which they are blended. From the test data I've read, the mileage difference is buried in the noise of the data, so your variation may just be on the high side of typical variance. But premium fuel certainly won't increase fuel mileage, as some believe.

The downside will be a lower resistance to knock, which will cause the engine to alter timing when knock-sensors detect it, slightly lowering peak horsepower. I don't have the exact numbers handy, but I recall that Lexus lowered both the octane recommendation and the horsepower rating of the engine in my daughter's car a couple of model years later, with no changes to the engine.

If you're on the track, at peak power for long periods of time, I'd recommend higher octane - for peak power/lap times/fun, and to avoid any issues with knock sensor overload (though I've never seen any data that backs up this concern). From an engineering standpoint, I believe using the minimum recommended octane rating is acceptable.

Last edited by PghRich; 06-08-2019 at 09:12 AM.
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Old 06-08-2019, 09:17 AM
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The simple answer is that Audi recommends 91 Octane.:


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Old 06-08-2019, 09:47 AM
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Originally Posted by CFLQTRO View Post
The simple answer is that Audi recommends 91 Octane.:


Often talked about on a lot of forums. Use what the car was designed to use. If it is designed for regular, there is no benefit in using premium. If it is designed for premium, use premium. Yes. Using regular in the A4 can cause damage.
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Old 06-08-2019, 10:44 AM
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Don't........................Leaded gas, Diesel, drive on rims. etc.
OK...........................87 AKI, Non top-tier gas, Drive on spare, etc.
Recommended........91, 93 AKI, top-tier gas, matching good rim and tire. etc,
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