I have seen several post that people claim to get better gas mileage using higher octane gas ie. premium over regular.

Is this true or are people/ vehicle computer being confused? Has anyone done a pen and paper calculation to truly calculate any benefit?

FTC claims there is no benefit.

The Low-Down on High Octane Gasoline

Just trying to clear up misnomer. I may need to send this to the guys over at Mythbusters LOL.

As it's been explained to me, by several people I trust, higher octane fuel holds less energy and therefore actually lowers fuel mileage.

In some cases this article is right; in some not-so-much. The main "player" is the ECM software and the parameters that the ECM can control. In the case of 4Runners (4th gens) the ECM has enough control that an MPG/Power benefit is available to those who are patient (4-5 tanks of fuel). The ECM 'listens' for knock and constantly 'smells' the exhaust among many other things. The ECM will advance the timing and lean the mixture to the parameters prescribed by the O2 sensors and/or it 'hears' the onset of knock. Higher octane fuel allows the timing to advance more and slightly leaner mixture. The inherent design of the engine can recover a bit more of the energy released by higher octane fuel. The slightly slower burn translates to the energy being recovered more efficiently by the piston/crankshaft. (That's PROVIDED there's enough 'room' within the ECM's parameter's.)

Some examples are the following paraphrased entries found in several owner's manuals "To achieve maximum mpg and power Premium Fuel is recommended." Toyota's FJC's, 2nd gen 3.slo's, GX470's and others have that entry.


Is there a point of diminishing returns with regard to higher octane? Absolutely. It's hard to know and is determined by many factors such as compression, stroke, ECM programming etc.



Anything that's as complex as the engineering/theory of modern engines can not be summed up by the Government, especially in one page.



Am I recommending Premium? No. If you want to fine and you may or may not gain something. In 4Runners I would say don't run 1-2 tanks and say it did nothing (to condemn the use of Premium). The 'learning' design of the Toyota ECM takes 4-5 tanks to learn how to properly digest the Premium fuel. Below is one of many summaries what I just summarized.

Welcome to Land Cruiser Owners On Line

My understanding is slightly less energy but more of it is "usable" if the engine is set up for it...

Higher octane fuels are a bit more stable and doesn't ingnite under certain temps and performs best in high compression or forced induction engines. With my modded turbo 1st gen DSM I could run 91 on the street but in the heat of a dragstrip enviroment and running 5 more psi I had to bump it up to 101 but anything less and it would detonate pretty easily under WOT.

Thanks CJ3Flyr..... So I guess the guys over at mythbusters would most def. bust that myth. There is no way they would extend a test for two plus months maybe two tanks max.

There is no real difference in the amount of energy between octane’s. The only difference is how each octane prevent engine knock. Depending on the engine design (Mostly compression) different octane will or will not prevent knock. The BEST octane to use is the lowest octane that will prevent knock. Any higher octane is a waste of money and will NOT increase mileage.

Koz

You're stating categorically that the ECM won't advance timing or lean the mixture in response to premium gas? What makes you so certain?

Exactly... Many ECU's, even through the late '90's, wouldn't take advantage of higher octane so the answer is, generally speaking, as Koz stated, use the lowest possible that doesn't knock. Today we're faced with many ECU's; some do Premium fuel, some don't, some adapt virtually instantly (Caddy Northstar's) and others that take a long time (our Toyota's).

Octane rating doesn't measure octane or energy content of a fuel, only its anti-knock properties.

Koz

I pour 91 octane (Shell). I get 17 in the city and 21+ (pending how fast i drive) on the freeway. To each own.

We bought a new Mini Cooper in the spring of 2009. I didn't realize it when I bought the car, but BMW recommends high test. We record the gas mileage of every tank, and the Mini has 16,000 miles on it now. We'll run 10 tanks of high test, then 10 tanks of medium test. There is no difference in the MPG figures.

Not seeing a connection between question and answer.

To me the question is not which gas grade provides more MPGs, but which one is more economical?

With that in mind, the premium grade gas would have to offer 10% to 15% improvement when compared to regular gas to make sense economically. I don't see that happening...

As far as premium fuel, more than enough octane will not trigger a timing adjustment. When the octane is too low the ignition system will sense the potential knock and adjust the timing to avoid damage. This is why using too high an octane is a waste. You need higher octane for engines that are specifically tuned to use it. Toyota/Lexus designs and tunes their modern engines right at point where higher octane is recommended. If you use an octane that is borderline and there is a high load put on the engine you may lose efficiency because the timing will be adjusted to prevent damage to the engine.

I think what confuses is the timing and air/fuel mix is also adjusted to compensate for different operating conditions such as temp, engine RPM, throttle position, etc.

Koz