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It is well understood that the engines in our vehicles are highly computerized and capable of managing a wide range of octane fuels. For me, the questions of which fuel to use boils down to two things. 1. Is it more economical to burn a higher octane fuel?, and 2. Does the vehicle perform better on higher octane fuel? From my initial testing I'd have to answer maybe. It's too soon to know with any degree of certainty, but my preliminary results look like it may be a yes.
MPG vs. Octane data in my 2014 RX350 AWD after around 8,000 miles. All fuel economy data is measured and not from the onboard computer. I emphasize that there are not enough data points to draw firm conclusions but my MPG did show a trend of improvement with higher octane fuel. But, higher octane fuel does not necessarily translate to lower cost per mile. The price of midgrade and premium fuel may be too high to make any possible higher MPG economically worthwhile. Today's gas prices in my area: Regular $3.39 Midgrade: $3.59 and Premium $3.79
Assuming these current fuel prices and that the very few tanks of midgrade and premium fuel I've used so far yielded reasonably accurate and repeatable fuel economy results (BIG if, simply not enough data points to have any degree of assurance):
A gallon of Regular fuel @ $3.39/gal yielded 23.7 miles or 14.3 ¢ a mile. (*recently getting 25 MPG)
A gallon of Mid-grade fuel @ $3.59/gal yielded 24.2 miles or 14.7 ¢ a mile.
A gallon of Premium fuel @ $3.79/gal yielded 27.4 miles or 13.8 ¢ a mile.
*my fuel economy has improved markedly after I hit 6,000 miles. I have no idea why. I have managed 25 MPG, measured, on the last two fill-ups using regular 87 octane fuel around town (small rural town). This throws a monkey wrench into the data I have collected so far and I may just wipe it and begin anew.
Intangibles are also worth consideration. If you notice smoother running, improved power, less downshifting, the use of premium may be something to consider even if it is more expensive. The cost to fill up with premium is only around $6 more than regular. And, if my preliminary test results hold up, it MAY possibly be less expensive to use premium fuel. Again, I stress that I've not run enough tanks of premium or mid-grade fuel through the vehicle to know if the results will hold over time and I have this unexplained improvement in my MPG at around 6,000 miles.
Helps especially if you are climbing big hills; my LEXUS is a different car with 91 in it and pulls the hills to Las Vegas very well with great gas mileage as it coasts well on the downside!
... I emphasize that there are not enough data points to draw firm conclusions but my MPG did show a trend of improvement with higher octane fuel. But, higher octane fuel does not necessarily translate to lower cost per mile...
Thanks for your analysis Jim. I look forward to following your MPG vs octane research as you put more miles on your RX.
My observation with my 2010 and 2012 Rx's has been that you get up to 2 miles per gallon better mileage with Premium. The 92 ctane costs at least $.40 per gallon more in the Chicago area which is roughly 10 to 12 percent higher. The mileage increase is also about 10 to 12 percent so in my opinion it is a "push" whether to use premium or not cost wise.
Jim's data is very interesting, but there's are just too many variables to give a clear definitive answer. "new" gas mixes with "old" gas, altering the actual octane., weather, different routes/traffic, driving counter to / or in sync with earth rotation, solar flares etc. I know I get better mileage when I wash my RX or eat a light breakfast.
Maybe buying gas from different stations or brand has a bigger effect.
In spite of that I usually fill with with 90+ every third tank or so. It "feels" like a more responsive ride. Also I assume there are more additives and cleaners.
Exactly, the amount of controls you'd have to establish in order to make this a valid argument makes this all moot.
We ignore the simple answer that we all know to be true about octane level. If you are not changing the compression ratio, then the engine does not care and will not know what octane level you are using. 87 will combust at the same level is always does in our engine. 89 will combust at 87 level. 91 will combust at 87 level. and so on.
mpg changes because of a truckload of other variables. octane level is not one of them.
Of course guys, this all depends on your cost of fuel. in my parts of FL. most gas stations post 87 or 93. nothing in between. and cost is very negligible. 5-7 bucks on a full tank. one less Ruffles chip bag a week of the grocery list.
On a side note, since i use my AC/cooled seats 80% of the time around the year, car moves better and acceleration is noticeable with seat coolers and AC at blast. Hence for me 93 is worth it.
well guy`s, for the time being i decided to stay with regular. i used premium in my 05 (cause that's all it ran good on) and i like NOT having to pay the extra money. runs good enough for me on 87.
Of course guys, this all depends on your cost of fuel. in my parts of FL. most gas stations post 87 or 93. nothing in between. and cost is very negligible. 5-7 bucks on a full tank. one less Ruffles chip bag a week of the grocery list.
That 5-7 bucks is the cost of charging my 2 EVs from low to Full battery using SCE special rates for EV ownerx.
The RAV4EV gets 103-140 miles of range and the S85 gets 265 miles
I guess it's all about perspective.
"We ignore the simple answer that we all know to be true about octane level. If you are not changing the compression ratio, then the engine does not care and will not know what octane level you are using. 87 will combust at the same level is always does in our engine. 89 will combust at 87 level. 91 will combust at 87 level. and so on."
We ignore this because it isn't true. Modern engines know and care what the anti-knock properties of the fuel are (the octane rating), and adjust the timing accordingly, extracting more power from higher octane fuels.
"We ignore the simple answer that we all know to be true about octane level. If you are not changing the compression ratio, then the engine does not care and will not know what octane level you are using. 87 will combust at the same level is always does in our engine. 89 will combust at 87 level. 91 will combust at 87 level. and so on."
We ignore this because it isn't true. Modern engines know and care what the anti-knock properties of the fuel are (the octane rating), and adjust the timing accordingly, extracting more power from higher octane fuels.
I have no problem learning something new.
Please share what this statement is based on because I've ever only heard it in reverse. I've never seen anything in print that says an engine designed for lower octane will re-engineer itself to be a better engine with more hp and more mpg because someone spends 40 cent more per gallon at the pump. Unless you change the compression ratio, the gas will combust at the lowest rated octane level unless that gas does not meet that minimum and thus "knocking" will occur.
Jim, When you stated your mileage went up at 6,000 miles. If that was with 6,000 miles total on the odometer, then the reason is that your engine was finally broken in. That is normal for them to wear in a bit and as parts fit more tightly for mileage to increase. This can occur for up to around 10,000 miles on some cars.
We have knock sensors that determined the octane being used and computers that adjusted the timing etc to compensate for regular, mid-grade or premium gas. Thus you should not have engine knock, but may notice reduced or increased power with different types of gas. Even my 1994 Mazda MX-6 had knock sensors.
Car and Driver did a gas test a few years ago, running Premium in a Honda Accord (and a few other cars) designed for regular. They also ran regular in some cars designed for Premium. See link. The Honda ran worse with Premium gas. http://www.caranddriver.com/features/regular-or-premium
Please share what this statement is based on because I've ever only heard it in reverse. I've never seen anything in print that says an engine designed for lower octane will re-engineer itself to be a better engine with more hp and more mpg because someone spends 40 cent more per gallon at the pump. Unless you change the compression ratio, the gas will combust at the lowest rated octane level unless that gas does not meet that minimum and thus "knocking" will occur.
Why do you believe this 3.5L v6 engine was designed specifically for 87 octane. The same engine is used in the RX450 and there calls for 91 octane and also produces a bit more horsepower with the identical 11.8:1 compression ratio. I'd be more apt to believe the 11.8:1 compression ratio was chosen to deliver optimum performance on 91 octane fuel and through the use of knock sensors and computer controlled detuning can operate on 87 octane fuel without damage.
I had my 04 RX330 for 10 years and performed many tests of fuel economy over the same routes. Around 18 expressway tests of 1500 miles each. I consistently got around 2 MPG better fuel economy when I used 91 octane fuel vs. 87 octane fuel. The 04 OM did specify that premium fuel would deliver better performance and it did. Back when I began this testing 91 octane fuel was 20 cents a gallon more expensive. Typically on a 16 gallon fill, I'd be spending an extra $3.20. I'd be able to get about 32 more miles per fillup (16 gal) using premium so it was less expensive to use premium than regular. Today the price difference for premium is 40 cents a gallon around these parts which obliterates economy as a reason for using premium.
I really don't have enough miles on the new 2014 to prove the point with this vehicle but so far the preliminary results suggest that premium may still deliver higher fuel economy.
08-05-14, 07:26 AM
MPG vs. Octane
