Octane Rating

[MikeM.]

And something else to take into account: ethanol in gasoline will yield lower mpg due to ethanol's lower energy content. So you should ensure that the ethanol content (if any) in the gas that you were buying is the same. I have heard that some companies may put more ethanol in their regular gas, but less in the premium.

I don't see why ethanol content needs to be taken into account if the goal is to determine which fuel provides the best MPG and drive-ability. It doesn't matter WHY one fuel is better, only that it is (or isn't). Cost might be a factor, average MPG might be a factor and how well it runs might be a factor but who cares if the fuel that comes out on top has more or less alcohol in it?

 

[IHeartGroceries]

There's just too many variables to make this experiment useful. There's too many variables in the type and quality of the fuel you are using (yes, even if from the same filling station), climate, road characteristcs/condition, and even in your driving habits (that are likely unnoticed).

There's really no point in running premium fuel in these cars.

And by the way, if you're in southern regions, you're already running summer (low RVP) gas in your vehicle. Official tank turning is deadlined in the next 2-4 weeks for most southern states. West and north are likely already getting into blend down.
You will lose efficiency during the transitional period, say, when you have low RVP in your gas tank <7.0, when there is all-of-a-sudden a cold spell (or, vise verse in the blend up season).

 

[MikeM.]

And by the way, if you're in southern regions, you're already running summer (low RVP) gas in your vehicle.

Of course the summer blend is just diluted into the winter blend so, depending upon the size of the tanks, the level of the tank when the summer blend was delivered and the sell rate of the fuel the mix will take some time to become mostly summer blend. Premium could very well stay closer to winter blend for a longer time due to it's lower volume of sales. It all depends upon the particulars mentioned above.

Official tank turning is deadlined in the next 2-4 weeks for most southern states. West and north are likely already getting into blend down.
You will lose efficiency during the transitional period, say, when you have low RVP in your gas tank <7.0, when there is all-of-a-sudden a cold spell (or, vise verse in the blend up season).

For those of us who fill up at stations in WA state, I do not believe the state requires a different summer blend. Summer blend costs more to make so I doubt suppliers would deliver summer blend unless it's all they could conveniently get.

 

[IHeartGroceries]

Of course the summer blend is just diluted into the winter blend so, depending upon the size of the tanks, the level of the tank when the summer blend was delivered and the sell rate of the fuel the mix will take some time to become mostly summer blend. Premium could very well stay closer to winter blend for a longer time due to it's lower volume of sales. It all depends upon the particulars mentioned above.



For those of us who fill up at stations in WA state, I do not believe the state requires a different summer blend. Summer blend costs more to make so I doubt suppliers would deliver summer blend unless it's all they could conveniently get.

You're definitely on to something - It takes more time/more blend down, reduced RVP batch receipts to fully turn a premium tank, because, you're exactly right, premium throughput if far less. In fact, it is often a challenge to turn premium tanks every year for this reason. They always seem to come down to the wire (deadline date). Believe me, climbing tanks and analyzing samples gets old.

Mandates and volatility regulations vary so greatly across the nation, and between states, counties, municipalities, etc, that I couldn't possibly keep up with them all. But, all 50 states are mandated by the EPA in regards to RVP. You can reference a regulatory "list" on the EPA's website, which includes dates and pressures for each state. WA definitely has to conform to a regulation (RVP cap), but you may be right about no actual blend up/down season (summer/winter blend). Areas with a more neutral climate, less emissions regulation tend to require less volatility regulation. Rural counties with conventional gas as well…

 

[MikeM.]

This chart explains the maximum RVP (Reid Vapor Pressure) limits and dates for various States/Counties of the US.

http://www.epa.gov/otaq/fuels/gasolinefuels/volatility/standards.htm

Interestingly, the summer blends generally contain the most btu's and return the highest MPG and these blends are the direct result of more stringent government regulations which often require a lower RVP than is allowed in the winter months.

So, for areas that have varying RVP limits for different seasons, the stricter summer limits not only provide improved air quality but also results in fuel that provides more range.

 

[CX-SV]

Now back from vacation, I feel like I can provide a bit better feedback on my experiement with Shell 93 octane fuel.

Let's start with fuel economy. While I am getting about 1.5 more mpg since starting the experiment, I must also report that the weather has warmed up nicely which also helps to improve gas mileage. I'm now getting 32 mpg versus about 30.5 mpg in mountain driving (80% highway) in my AWD model. Given the improving weather, I don't know that I can draw any firm conclusions.

We'll cover power next. Please don't expect an objective report from me. I don't have all the tools necessary for a scientific report, so I will report only on my impressions...Low end torque does seem a bit better -- just a perception of easier accereration in first and second. And for some reason, I do feel a bit more confident when merging at highway speeds, so perhaps there is some advantage at higher RPMs as well. I can't say that my experience has been transformed, but there does seem to be some modest improvement for me.

My last impression will be regarding engine noise, which I believe sounds slightly smoother and maybe not quite as buzzy as before. Again, this is a subjective evaluation

While I agree that the 2.0 CX-5 will run just fine on 87 octane, and that the engine is programmed to handle 87 without problems, it does seem logical to me that 13:1 engine might modestly benefit from better gas, once the computer recognizes a higher quality product and makes the necessary adjustments. I have determined, in my own subjective way, that there is some modest benefit. Since we really aren't talking about huge amounts of money, I'm completely happy with my decision to stick with 93, at least for a while.

Thanks for report, are you still using premium gas?

 

[frz]

Hi guys, I hope everybody here who cares about mileage and such is tracking the fuel economy with hard numbers. I've been using fuelly.com and 89 octane, averaging about 9.5L/100km from full to empty each time. More data points would be great.

 

[MikeM.]

Hi guys, I hope everybody here who cares about mileage and such is tracking the fuel economy with hard numbers. I've been using fuelly.com and 89 octane, averaging about 9.5L/100km from full to empty each time. More data points would be great.

It would be really great if you put your Fuelly badge in your sig so we can all see what you're talking about in units that are more familiar in our various locals.

 

[sylva143]

Thanks for report, are you still using premium gas?

Yes, I'm still on 93 octane. My mileage has risen to 33.6 mpg. I don't have any additional changes in performance or sound, other than what was reported in my last post. I've thought about switching back to 87 for additional testing, but so far I'm satisfied enough with my experiment to continue.

Gas prices have also come way down around here, so I'm now paying for premium what I paid for regular a couple of months ago.

 

[CX-SV]

Yes, I'm still on 93 octane. My mileage has risen to 33.6 mpg. I don't have any additional changes in performance or sound, other than what was reported in my last post. I've thought about switching back to 87 for additional testing, but so far I'm satisfied enough with my experiment to continue.

Gas prices have also come way down around here, so I'm now paying for premium what I paid for regular a couple of months ago.

I see and probably the increase in gas mileage is due to change to summer blend gasoline (w/significant impact on MPG), not to mention differences is driving conditions.

Regardless of fluctuations in gasoline prices, premium fuel will generally be 5% more expensive in most US markets. Expect gasoline prices to decline another 10 or 20 cents in next few months.

 

[sylva143]

I'm sure this update may cause some controversy, but after upgrading to a 2014 GT from a 2013 sport, I ran across a very interesting entry in the edmunds.com long term CX-5 test:

Article on Octane Rating

This entry has an imbedded article to another test that edmunds conducted on a Chevrolet Curze that indicated better mileage with 93 octane, under specific conditions.

After two tanks of 87 octane in my new GT, I was averaging about 29.8 mpg on my regular commute. After upgrading to 93 on my last fill-up ($.24 cent difference per gallon), my commute mpg rose to 32.0. Of course, I'll want to give it another week or so before making any final conclusions for myself. Other observations are a feeling of more fluidity in power and a better sounding engine upon acceleration.

I read the owner's manual as stating that 87 octane is the MINIMUM rating for the CX-5. It doesn't say anything about it being the best octane rating for mileage and performance. I read this as the minimum rating required to prevent knocking and engine damage.

After doing some reading on this topic, I believe that conventional wisdom has been correct -- that engines designed for 87 octane would find no benefit from higher octane gas (except for the added detergents). With manufacturers desigining higher-tech engines with high compression ratios, they have found ways to modify engines and exhaust to allow 87 to be used without complaint. They have also designed engines to adapt to different fuel octane ratings. As edmunds.com explains, there may be reasons to re-think the 87 octane mantra for the latest technology.

I'll continue to monitor my mpg and will provide updates...

 

[backpackerx]

2 points to think about here.

1. My MPG kept increasing up to about 3,000 miles which I'm attributing to break in so it may be early to test fuels.
2. The percent of increase in mpg has to be more than the percent increase in higher octane fuel cost over 87 for it to be beneficial from purely an economical perspective.

You can do a quick percentage increase compare, or do a simple division problem to see the cost in cents per mile by plugging in your own numbers.
$3.40 29.8 mpg = 11.40 cents per mile
$3.64 32 mpg = 11.38 cents per mile

I'm using my area's cost for 87 octane here and applying your .24 cent premium. In your case, if the mpg increase spread holds, you come out ahead. In my case, premium is .35 cents more than 87 so with those same mpg numbers I wouldn't benefit financially.

$3.40 29.8 mpg = 11.40 cents per mile
$3.75 32 mpg = 11.72 cents per mile

 

[IHeartGroceries]

Octane rating simply has no direct effect on efficiency at all. The quality of the gas does, which has a tendency to be better with premium fuels, because they are better protected by shippers, wholesalers, bulk storage, etc.. If one has an thorough understanding of how modern car engines function, they will arrive at this conclusion.

There's a million factors which can skew results in a very uncontrolled test, such as the one embedded in the article, with the Chevy Cruz. It simply is not conclusive.
And I am willing to wager that Chevy has some pretty aggressive timing tables in their ECU logic, set on by the detection of knock, in addition to (and especially with factoring in DI engine design) dumping of excess fuel into the cylinders, in order to cool the air charge, and further reduce knock onset. Meaning, any improved efficiency is resulted indirectly, and will vary significantly, based on climate, engine design, driving habits, fuel quality (region), etc.

But, I won't believe premium shows real efficiency improvements until I see some data out of a laboratory.

 

[sylva143]

Octane rating simply has no direct effect on efficiency at all. The quality of the gas does, which has a tendency to be better with premium fuels, because they are better protected by shippers, wholesalers, bulk storage, etc.. If one has an thorough understanding of how modern car engines function, they will arrive at this conclusion.

There's a million factors which can skew results in a very uncontrolled test, such as the one embedded in the article, with the Chevy Cruz. It simply is not conclusive.
And I am willing to wager that Chevy has some pretty aggressive timing tables in their ECU logic, set on by the detection of knock, in addition to (and especially with factoring in DI engine design) dumping of excess fuel into the cylinders, in order to cool the air charge, and further reduce knock onset.

I won't believe it until I see some data out of a laboratory.

Thanks for the feedback. I will continue the test for another week or so. So far, I've had the same experience with the 2.0 and now with the 2.5 -- a sudden jump in mpg. I know that I am getting good quality gas with the Shell V-Power. I'm just wondering if the point of the edmunds article was to question if there may be a new paradigm with octane and engines with the latest technology. I just find it curious that owner's manual specifies 87 as the mininum octane rating but not the required rating.

 

[RedBaron]

Octane rating simply has no direct effect on efficiency at all. The quality of the gas does, which has a tendency to be better with premium fuels, because they are better protected by shippers, wholesalers, bulk storage, etc.. If one has an thorough understanding of how modern car engines function, they will arrive at this conclusion.

There's a million factors which can skew results in a very uncontrolled test, such as the one embedded in the article, with the Chevy Cruz. It simply is not conclusive.
And I am willing to wager that Chevy has some pretty aggressive timing tables in their ECU logic, set on by the detection of knock, in addition to (and especially with factoring in DI engine design) dumping of excess fuel into the cylinders, in order to cool the air charge, and further reduce knock onset. Meaning, any improved efficiency is resulted indirectly, and will vary significantly, based on climate, engine design, driving habits, fuel quality (region), etc.

But, I won't believe premium shows real efficiency improvements until I see some data out of a laboratory.

Well, not sure that *I* would necessarily arrive at this conclusion . The fact is that the primary result of lowering octane is an increase in detonation (knock) and the primary way to overcome detonation is decreasing the leanness of the fuel mix. Modern engines have knock sensors to register this and feedback loops (as you state) to control it by making the fuel mixture more rich. Simplistically: lower octane, more knock -> more fuel, worse mileage. (Yes, I know that valve timing and other controls have an effect too, but the main premise stays.)

Now, that's the theory, but having said that I'm not sure exactly how much (or even if) it really, measurably, affects mileage. I have to say that about 10 years ago, I had an Audi A6 bi-turbo. Premium gas was 'recommended' but not enforced. I tried a number of tankfuls of regular vs. a number of premium and noticed little or no mileage differences. Now that was a more primitive engine, sensors and ECM (mid-90's development) than recent designs, and doubtless had far less compute power to handle the differences in fuel/octane/knock.

So bottom line, who knows? Like you, I'd love to see results of fully controlled testing.

 

[IHeartGroceries]

Well, not sure that *I* would necessarily arrive at this conclusion . The fact is that the primary result of lowering octane is an increase in detonation (knock) and the primary way to overcome detonation is decreasing the leanness of the fuel mix. Modern engines have knock sensors to register this and feedback loops (as you state) to control it by making the fuel mixture more rich. Simplistically: lower octane, more knock -> more fuel, worse mileage. (Yes, I know that valve timing and other controls have an effect too, but the main premise stays.)

Now, that's the theory, but having said that I'm not sure exactly how much (or even if) it really, measurably, affects mileage. I have to say that about 10 years ago, I had an Audi A6 bi-turbo. Premium gas was 'recommended' but not enforced. I tried a number of tankfuls of regular vs. a number of premium and noticed little or no mileage differences. Now that was a more primitive engine, sensors and ECM (mid-90's development) than recent designs, and doubtless had far less compute power to handle the differences in fuel/octane/knock.

So bottom line, who knows? Like you, I'd love to see results of fully controlled testing.

I definitely agree that premium fuel use has the potential for improved efficiency, by indirect means, such as the workings of feedback loops and ECU logic/parameters. But, I'll never buy into blanket statements which insist premium will net efficiency gains across the board. I will maintain that it is likely not worth the added expense to fill with premium, especially if you're doing so with high dollar brands, such as Shell's V-Power.

Thanks for the feedback. I will continue the test for another week or so. So far, I've had the same experience with the 2.0 and now with the 2.5 -- a sudden jump in mpg. I know that I am getting good quality gas with the Shell V-Power. I'm just wondering if the point of the edmunds article was to question if there may be a new paradigm with octane and engines with the latest technology. I just find it curious that owner's manual specifies 87 as the mininum octane rating but not the required rating.

I certainly won't fault you for experimenting. In fact, thanks for the feedback.
Shell is great fuel. Rather, their additive and treat rates are superior. The blendstock is likely always different, due to exchange business and marketing.
The key to this experiment is to establish as much control as possible, by buying from the same filling station, and ensuring that the filling station maintains high levels of throughput.

 

[CX-5_MAZDA]

Actually Mazda selected the 13:1 ratio for US market so that regular fuel could be specificed (they didn't have to, they chose to), so main impact of running higher octane premium fuel will be felt in wallet, not noticeably increased horsepower.

In Europe Mazda uses a 14: ratio and specifies a higher octane premium fuel requirement, and horsepower is reported as higher too.

This is correct. In Europe they run 14:1 because people there don't complain about having to use more expensive fuel (why that is I don't know). 14:1 engine has a better thermal efficacy/ better fuel efficacy (less of the fuels energy is lost as heat wastage) but if they use lower octane over there, they my cause detonation inside the cylinders instead of a 'controlled burn'.

 

[IHeartGroceries]

Because they have public transportation options!

 

[sussig]

so i just took my 2013 cx5 back to the dealership for a check up due to engline light came on. per dealer, cars with direct-injected engine such as our cx5 requires name brand 87s ? although it doesnt say that in the owner's Manuel, only that 87 and up and i have been putting in 87 gases from costco, sam's and von's owned gas stations. Dealer said those gas stations are craps and it caused the carbon built up on my fuel injectors and it had to be replaced. Nevertheless, my car is still under warranty and dealer has replaced 1 fuel distributor, 12 grommets, 4 o rings, 4 gasket inlet manifolds, 1 induct kit, 4 fuel injectors. these replacements would have costed $1600 if car was outta warranty.

Now, i have no problem puttin in name brand 87s, however, i cant stop thinking if mazda did a false advertising on the fuel type needed in cx 5. Its never mentioned that cx 5 requires specific name brand gas to run smoothly.

 

[metalfab]

When I had a 2012 RAM 1500 with a 5.7L HEMI (390HP / 407 Torque). I used the recommended 89 octane. Then tried 87 (made a slight knocking on hills), also 91 Shell which has no Ethanol and even Chevron 94 which doesn't have Ethanol. I saw at best 3% difference in better fuel mileage going from 89 to 91/94. That could have been a tailwind for all I know. Save your money, use what is stated on the door as everything else is a placebo effect at best.