Choosing octane ratings

I have over the years read numerous articles regarding choice of fuel octane for proper and efficient operation of one’s auto. The " opinions" range from " you are wasting money by using a higher octane than recommended" ,to " if you hear a persistent pinging then a higher octane may correct the problem" , to " it is not a concern since modern engine systems have computer systems that make octane adjustments.

I own 2005 Toyota Highlander (3MZ-FE engine) and the manual states, " Select Octane Rating 87 (Research Octane Number 91) or higher. For improved vehicle performance , the use of premium unleaded with Octane Rating of 91 ( research Octane Number 96) or higher is recommended" This statement prompts several questions. What is Research Octane compared to " consumer octane" ? And, what is " improved vehicle performance" ? Also, most gas stations offer " three" choices of Octane. Why?, when most manuals recommend either “regular” or “premium”.

I did experiment using the highest available octane on a recent 600 mile trip and did gain an additional two miles per gallon. The cost per mile difference was only a few pennies and would likely be consistent with ones driving habits.

Since there is only a modest improvement, if any, in mileage benefit, could there be other benefits of “wear and tear” to be considered ?

I may miss your response if you publish an answer although , I often seek your column in the Washington Post. Is it possible to EMAIL so I will not miss you ?

Bob M — Nellysford, VA.

Bob M again : where does E 85 GAS fit in this question?

In non-high altitude areas it usually goes Regular = 87 octane, Mid-Grade = 89/90 and Premium is 91 octane or better. The engine in your car will run on 87 octane, however Toyota “recommends” using premium for added power and fuel economy. Some vehicles do recommend mid grade (although they are few and far in between) The newer Chryler Hemi powered cars and trucks recommend 89 octane.

Octane ratings tell us how easily a fuel explodes as opposed to buring. There are two common ratings for octane: Resarch Octane Number (RON) and Motor Octane Number (MON). Most of the pumps that I have seen show an average of these two numbers ((R+M)/2).

When an engine knocks (fuel exploding instead of burning), more engergy is lost to heat and noise than when the engine is running normally. Some cars have “knock sensors” that will retard the ingnition if it hears the engine knocking. Retarding the ignition causes the enginge to run less efficiently, but saves wear on the engine by preventing the knocking. As long as this knock sensor is working, you probably wouldn’t see much difference in the “wear and tear” of the engine. If you start to hear some knocking, then you would need to get the sensor repaired or switch to a higher octane rating for “wear and tear” reasons.

E85 is much different. It is 85% ethanol - 15% gasoline. The octane number is high, like above 100, but burns differently. It is a slower burn, so timing is usually advanced to compensate. Plus, it has less energy than gasoline. That typically means you need to burn more to give you the same amount of power. May people experience drops in gas mileage like 5-10%. If your car is not certified ‘Flex-Fuel’, the fuel system has not been hardened to resist damage for the much more corrosive properties of ethanol. In a non-Flex-Fuel car, E85 can cause damage in the fuel system.

As for gasoline, your choice, but the manual states your SUV will run just fine on 87 octane, but was tuned for optimal performance using 91 or better. You proved that yourself. Maybe 89 octane will give you the best of both worlds, better fuel mileage and decent cost at the pump.

Regular (87 octane) is fine, you need to calculate if premium (91 octane) is worth the money. Compare cost/mile = gas price/mpg. If it’s higher with premium, use regular.

The manual for my wife’s 2006 Sienna (3.3 liter, V6) says the same thing about gas. Last year I used premium exclusively for a few months and compared the mpg with the same time period the previous year when we were using regular gas. The mpg was the same for both grades of gas. Here’s the link for more detail

I never noticed any pinging with regular gas. It might be more of an issue if my wife and I had a lead foot.

Ed B.

You probably have noise induced hearing loss,I do, can’t hear high frequency,really it’s just a guess.

Is there not a limit on how far knock sensors will retard timing?

You answered your own question, for your vehicle. Your vehicle fairs best on high octane fuel; so, what’s the problem? It’s rather immaterial whether certain octanes are better for other vehicles, isn’t it? If not, where is the materiality?

My mom drives a '02 Nissan Murano, which recommends 91 octane. Note that there is no observed knocking/pinging on any gas used.

Considering the effect of high compression ratios on octane requirements (high CR=high octane), and that the air/fuel mix is compressed adiabatically (the lower the temp of the uncompressed air, the lower the final temp), I’ve ventured to her that she “needs” the good stuff normally, but probably can get by with 89 octane during the winter.


91 octane is recommended; so, use 91 octane. The seasons don’t change the octane needs. What happens inside the combustion cylinders is independent of the ambient temperatures.

I wonder if driving style dicatates the need for suggested premium vs regular. For example if you floor the throttle or use in hard usage(loaded vehicle up steep inclines on highway) do you end up with better MPG using premium since utilize its benefits vs a motor running on regular in hard conditions retarding itself to compensate for lesser fuel when it needs the extra power.