Octane effect on MPG

[Burt]

I have over 100,000 miles on my 2011 RX350 and have the following observations which are completely counterintuitive. Manual says 91 premium fuel is required, but there is ample evidence of people going 300,000 plus miles on this model with regular 87 octane and I'm hard pressed to find real world examples of damage. Here is what I find for driving 72-3 mph on interstate highways with wind not a factor:

On premium 93 octane: 23 mpg. The engine definitely has more torque and idles smoothly.
On regular E10 87 octane: 23.5 mpg. Notice minor loss of power and idle isn't as smooth
On Regular E15 88 octane: 24.2 mpg!

To me this makes no sense, but I swear it is what I've consistently seen. I thought it could be that premium gas has lower energy content (Btu/gallon) than regular but a refinery guy on BITOG provided documentation that this is not the case. And E15 has to have the lower btu content than the others but it gives the best gas mileage and is lowest cost. Just seems to be a sweet spot.

You would think my turbocharged EcoBoost Fiesta would benefit from higher octane even though it isn't required, but I can't say I can see a difference. I had a 2002 Nissan Maxima that recommended premium, but I had similar experience as with the RX.

Any technical explanation how this could be?

 

[KYMISTWALKER]

This is interesting the octane police will be here soon, I always run 93 octane in my smaller engines generators ATVs pressure washers

 

[Bob in Utah]

Wife’s ‘11 GX 460 suggests 91 in manual

We have readily available 88 eth free…which is what she uses

Truck will turn 20 mpg on regular basis…4.6L with 300 horse..full time 4x4

No issues…runs good sounds good…we live at 6K’

 

[fantastic]

How many samples of each did you use the individual octane?

 

[k1xv]

Of course, these fuels have different prices at the pump. For we cheapskates, the relevant measure is miles per dollar spent on fuel.

What is the pump price of E10 87 versus E15 88 ??

 

[2005jettatdi]

Chemically, lower octane has more energy.

 

[Burt]

How many samples of each did you use the individual octane?

This was not a rigorous, structured experiment, but rather observations over a long term. We drive 1,000 mile trips from Midwest to Texas each year. Sometimes I start out with a tank of premium and switch to regular. Sometimes it's the opposite. I truly and easily get the best mileage on E15 and try as I might do the worst on 93.

 

[Burt]

Chemically, lower octane has more energy.

I thought that was the case. What they add for octane lowers the btu content. Setting aside the contrary documentation a refinery guy sent me, we know that ethanol has higher octane and lower btu. Same for butane and pentane.

You'd think that E15 is lowest btu content, but maybe it is spiked with some high btu, low octane components.

 

[Burt]

What is the pump price of E10 87 versus E15 88 ??

15 cents per gallon lower than E10
Premium is 80 cents more than E10

95 cents per gallon lower price and 5% better fuel economy means 4.3 cents per mile lower operating cost or $4,400 savings over the first 100,000 miles.

 

[Hermann]

Several times in the past 5 years since E88 became available here I have got between 1-2 mpg better.

 

[BMWTurboDzl]

I have over 100,000 miles on my 2011 RX350 and have the following observations which are completely counterintuitive. Manual says 91 premium fuel is required, but there is ample evidence of people going 300,000 plus miles on this model with regular 87 octane and I'm hard pressed to find real world examples of damage. Here is what I find for driving 72-3 mph on interstate highways with wind not a factor:

On premium 93 octane: 23 mpg. The engine definitely has more torque and idles smoothly.
On regular E10 87 octane: 23.5 mpg. Notice minor loss of power and idle isn't as smooth
On Regular E15 88 octane: 24.2 mpg!

To me this makes no sense, but I swear it is what I've consistently seen. I thought it could be that premium gas has lower energy content (Btu/gallon) than regular but a refinery guy on BITOG provided documentation that this is not the case. And E15 has to have the lower btu content than the others but it gives the best gas mileage and is lowest cost. Just seems to be a sweet spot.

You would think my turbocharged EcoBoost Fiesta would benefit from higher octane even though it isn't required, but I can't say I can see a difference. I had a 2002 Nissan Maxima that recommended premium, but I had similar experience as with the RX.

Any technical explanation how this could be?

Octane recommendations are for maximum performance (HP/TQ) with a negligible impact on MPG.

 

[RDY4WAR]

There is a measurable difference in fuel economy with my '25 F150 with the 3.5L ecoboost. On the same route (all highway), similar weather and traffic, and cruise control set at 70 mph to eliminate driver input, premium 93 gets +1.5-2.0 mpg better than regular 87 in my A-B-A-B testing. It averages ~21.5 mpg on 87 and ~23.3 on 93. That's measured based on gallons to fill vs miles using the same gas pump.

EDIT: I tested both 87 and 93 for ethanol content and both were 9-10%.

 

[K-Man]

The SOHC 4.0 in the old Ford Rangers runs better and gets better fuel economy when using premium 93–octane. The Ranger forums have discussed this a great deal. The SOHC was designed in Germany for the gasoline in use there, which is higher octane than standard US fuel, but Ford put a knock sensor on the engine and told American owners using regular 87–octane is all right. Not so fast. This means the sensor detects knocks and is retarding the timing all the time, affecting fuel economy.

On 87–octane I get 2–4 miles/gallon less than on 93–octane on trips in my 2011 Ranger with that engine. On long trips at lower speeds (55–60 mph tops) I have achieved as much as 24 mpg on premium.

 

[grasscutter]

I think it's the ethanol content that affects the fuel mileage the most.

For some reason, I lose 3 miles per gallon after I filling up at Costco.

I have since switched to Sinclair. Fuel mileage is much better.

 

[tmorris1]

Chemically, lower octane has more energy.

Maybe so with straight gasoline, but I think the 10 and 15% ethanol of 87 and 88 octane will lower the energy.

 

[Astro_Guy]

Maybe so with straight gasoline, but I think the 10 and 15% ethanol of 87 and 88 octane will lower the energy.

Ethanol actually increases octane, so maybe the E15 starts with an even lower octane base fuel.

 

[k1xv]

Back in the 1970 era there was an SCCA racing class, Formula Ford. All the cars had engines that were as identical as possible. Back in those days, gasoline was gasoline. Mostly leaded gas, with no ethanol. It was generally thought that regular grade gasoline had more BTU per gallon than premium, and the savvy drivers adjusted their carburetors and ignition timing to run on regular. I don't know if it really gave them any edge.

But adding ethanol to gasoline definitely reduces the BTU per gallon, and from the statistics I have seen, regular grade gasoline is likely to have more ethanol than premium, even if both are labeled st the pump as "up to 10% ethanol".

 

[PontiacHO]

There is a measurable difference in fuel economy with my '25 F150 with the 3.5L ecoboost. On the same route (all highway), similar weather and traffic, and cruise control set at 70 mph to eliminate driver input, premium 93 gets +1.5-2.0 mpg better than regular 87 in my A-B-A-B testing. It averages ~21.5 mpg on 87 and ~23.3 on 93. That's measured based on gallons to fill vs miles using the same gas pump.

EDIT: I tested both 87 and 93 for ethanol content and both were 9-10%.

When the engine retards timing due to lower octane (spark knock) you’re going to get less fuel mileage than higher octane on engines that require higher octane (more compression - turbo & blower boost)…

 

[IndyIan]

I have over 100,000 miles on my 2011 RX350 and have the following observations which are completely counterintuitive. Manual says 91 premium fuel is required, but there is ample evidence of people going 300,000 plus miles on this model with regular 87 octane and I'm hard pressed to find real world examples of damage. Here is what I find for driving 72-3 mph on interstate highways with wind not a factor:

On premium 93 octane: 23 mpg. The engine definitely has more torque and idles smoothly.
On regular E10 87 octane: 23.5 mpg. Notice minor loss of power and idle isn't as smooth
On Regular E15 88 octane: 24.2 mpg!

To me this makes no sense, but I swear it is what I've consistently seen. I thought it could be that premium gas has lower energy content (Btu/gallon) than regular but a refinery guy on BITOG provided documentation that this is not the case. And E15 has to have the lower btu content than the others but it gives the best gas mileage and is lowest cost. Just seems to be a sweet spot.

You would think my turbocharged EcoBoost Fiesta would benefit from higher octane even though it isn't required, but I can't say I can see a difference. I had a 2002 Nissan Maxima that recommended premium, but I had similar experience as with the RX.

Any technical explanation how this could be?

Advancing timing to where knocking just starts, increases HP/efficiency with any octane of gas.
My limited understanding is that using 87 octane gets your vehicle knocking or very close to knocking more often, so its kind of like advancing the timing a little bit. Toyota likely is a bit conservative on the timing advance tuning with 91 octane, so dropping the octane down gets the engine running with a bit more efficiency with the same timing tuning.

 

[y_p_w]

When the engine retards timing due to lower octane (spark knock) you’re going to get less fuel mileage than higher octane on engines that require higher octane (more compression - turbo & blower boost)…

There’s going to be a combination of timing advance and energy content. If the energy content is identical, higher octane rating should result in better fuel economy if the timing can be advanced for higher efficiency.