Better gas mileage with Super octane?

[Yousuf Khan]

My car is an 2000 OBW, with a 2.5L H4, recommended gasoline is just
regular (87 octane). Now I've read in magazines and online that cars
designed for regular can't really take advantage of higher octanes, so
you shouldn't even bother to put higher octane fuel in, it's just a
waste of money. Now cars designed for higher octane can take lower
octane at the disadvantage of reduced power as the engine computer dials
back the tuning to prevent knocks and pings. But cars with regular as
their standard don't have computers that dial up their tuning.

So anyways, I decided to try out some super octane the other day. I
filled half the gas tank with it (I already had a half-tank of regular
already in there). I would say the average octane in there was a
mid-grade 89 (half regular + half super). Then I took my regular long
ride (200km). I tend to get 28 mpg usually on this ride, but I got 31
mpg this time. Now I wasn't doing anything differently than I usually
do, I set to cruise to 125 km/h (around 80 mph).

Now just to avoid some confusion, when I talk about gallons, I'm talking
about Canadian Imperial Gallons (4.5 litres), not US Gallons (3.75
litres). But the relative differences are easily understandable.

So why would this be the case? I've heard other people in other cars
talk about this too.

Yousuf Khan

 

[Josh Assing]

You should not base it on one drive -- the octane improvement based on 1/2 tank
mixed with 1/2 tank would be negligable.

MPG should be based on many many tanks. Traffic, Tempurature, MINOR variations
in speed, throttle, gear shifts etc would effect millage.

"tend to get 28" that implies sometimes you get more sometimes less.

a 2000 OBW does have a computer that keeps the mixture optimized.

My car is an 2000 OBW, with a 2.5L H4, recommended gasoline is just
regular (87 octane). Now I've read in magazines and online that cars
designed for regular can't really take advantage of higher octanes, so
you shouldn't even bother to put higher octane fuel in, it's just a
waste of money. Now cars designed for higher octane can take lower
octane at the disadvantage of reduced power as the engine computer dials
back the tuning to prevent knocks and pings. But cars with regular as
their standard don't have computers that dial up their tuning.

So anyways, I decided to try out some super octane the other day. I
filled half the gas tank with it (I already had a half-tank of regular
already in there). I would say the average octane in there was a
mid-grade 89 (half regular + half super). Then I took my regular long
ride (200km). I tend to get 28 mpg usually on this ride, but I got 31
mpg this time. Now I wasn't doing anything differently than I usually
do, I set to cruise to 125 km/h (around 80 mph).

Now just to avoid some confusion, when I talk about gallons, I'm talking
about Canadian Imperial Gallons (4.5 litres), not US Gallons (3.75
litres). But the relative differences are easily understandable.

So why would this be the case? I've heard other people in other cars
talk about this too.

Yousuf Khan

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[Dominic Richens]

My car is an 2000 OBW, with a 2.5L H4, recommended gasoline is just
regular (87 octane). Now I've read in magazines and online that cars
designed for regular can't really take advantage of higher octanes, so you
shouldn't even bother to put higher octane fuel in, it's just a waste of
money. Now cars designed for higher octane can take lower octane at the
disadvantage of reduced power as the engine computer dials back the tuning
to prevent knocks and pings. But cars with regular as their standard don't
have computers that dial up their tuning.

So anyways, I decided to try out some super octane the other day. I filled
half the gas tank with it (I already had a half-tank of regular already in
there). I would say the average octane in there was a mid-grade 89 (half
regular + half super). Then I took my regular long ride (200km). I tend to
get 28 mpg usually on this ride, but I got 31 mpg this time. Now I wasn't
doing anything differently than I usually do, I set to cruise to 125 km/h
(around 80 mph).

1. the higher the octate the lower the energy content of the fuel, so all
things being equal, you get less miles per gallon with a higher octane fuel.
2. 200km is about 16 litres of fuel - how are you able to measure this
accurately?
2. you need more than a sample of 1 to be statistically significant.

 

[Ed P]

1. the higher the octate the lower the energy content of the fuel, so all
things being equal, you get less miles per gallon with a higher octane fuel.
2. 200km is about 16 litres of fuel - how are you able to measure this
accurately?
2. you need more than a sample of 1 to be statistically significant.

I agree with DR's answer above but would add that, if there is any
improvement in mpg, it is because the higher octane is allowing the
engine computer to use more timing advance. The question then becomes
whether any money savings from any improvement in mpg exceeds the extra
cost for the higher octane fuel.

Ed P

 

[Carl 1 Lucky Texan]

I agree with DR's answer above but would add that, if there is any
improvement in mpg, it is because the higher octane is allowing the
engine computer to use more timing advance. The question then becomes
whether any money savings from any improvement in mpg exceeds the extra
cost for the higher octane fuel.

Ed P

You got close to my question; Yousuf, did you reset the ECU before the
test to force readjustment of the engine timing? Even so, it could take
severla 'drive cycles' before the ECU makes the proper changes. Doing
this immediately before a long trip(did you happen to increase the tire
pressure, replace the air filter or make any other changes?), with no or
limited 'on-off' or 'warm up/cool down' engine cycles is not a
reasonable test. Also, the expensive gasoline might also have more
detergent in it or some other additive. Plus, warmer weather can result
in an engine needing a little more 'headroom' as concerns octane, such
that running low octane in winter and higher octane in summer could be
advantageuos.
A better test might be to fill with mid grade, clear the ECU, and
monitor about 6 tankfuls purchased from the same pump to see if there is
an improvement equal to or better than the cost increase.

interesting

Carl

 

[B A R R Y]

FWIW, My '05 Tacoma has 91 octane as a "recommended" fuel, but can run
on 87. Rumor has it that the original manual required premium, but it
was reworded when gas prices shot up.

I ran 10 consecutive tanks of each octane and tracked the cost and MPG
in an Excel spreadsheet. The per-mile cost is virtually identical, with
a maybe a millicent nod to premium. I get an MPG gain with $0.25-0.30
more expensive premium. The truck performs slightly better on premium
(a tad more pickup, no difference in cruise), and definitely better on
hot days and when towing. The 10 tanks of regular were run pre-test,
and the data was pulled from Quicken. I track MPG data in the
transaction comment field. Obviously, fuel cost is always part of the
transaction. <G>

I didn't reset anything, as I figured 10 tanks would be enough for the
ECU to adjust itself. I typically run all the way down to the warning
light before refueling.

I think this would be a worthwhile experiment in many vehicles. I would
also bet the vehicles with no mention of higher octane in the manual
would see little or no benefit. As gas prices climb, the numerical cost
of premium seems to stay the same number of cents above regular, making
the relative cost difference, as a percentage, cheaper.

 

[YKhan]

MPG should be based on many many tanks. Traffic, Tempurature, MINOR variations
in speed, throttle, gear shifts etc would effect millage.

No, I understand that, and I'm not saying this is a scientific test. I
kept my driving "style" the same on this trip. I can't account for
slight variations in traffic, temperature, etc. But I can account for
throttle and gear shifts to a certain extent, because I usually set the
cruise control once at speed, and keep it on cruise control for as long
as possible. I've learned that the cruise control is my best friend,
not only to relieve fatigue in my legs, but also I get better gas
mileage with it, because I don't tend to go up and down the speed
range, I stay steady-state on a single speed for hours on end; also I
tend to have a better average speed when cruise control is on, again
because I'm not going up and down the speed range.

"tend to get 28" that implies sometimes you get more sometimes less.

Yes, but it's pretty consistently around 28 mpg when I'm cruising at
125 km/h, the variations would tend to be less than 0.5 mpg at that
speed. In the past, I've also experimented with higher and lower
cruising speeds. Whenever I've cruised upto 130 km/h, I would lose
about 1-2 mpg. Whenever I've cruised down at 120 km/h I might be
getting closer to 29 mpg. Going all of the way down to 115 km/h, I
would then tend to get over 30 mpg (usually 31 mpg). But I don't have a
lot of patience for 115 km/h, so my usual speed is 125 km/h.

So actually I got close to the same mileage I would tend to get with
regular at 115 km/h but using the higher octane doing 125 km/h.

a 2000 OBW does have a computer that keeps the mixture optimized.

Yes, but my understanding is that it can adjust things to optimum at 87
octane, but it can't adjust to optimum for higher octanes, it just
keeps it at the same setting as it sets for 87 octane. From sources
I've read, they say that a car designed for 87 octane won't readjust
itself for higher octanes; there are no engine mappings inside the
computer beyond 87 octane. However, cars designed for the higher
octanes can readjust themselves downward for lower octanes.

Yousuf Khan

 

[YKhan]

1. the higher the octate the lower the energy content of the fuel, so all
things being equal, you get less miles per gallon with a higher octane fuel.
2. 200km is about 16 litres of fuel - how are you able to measure this
accurately?

Actually it's about 19 or 20 litres. Remember I'm talking imperial
gallons (4.5 L) here.

How do I measure it? Simple, I fill up before leaving on the trip, and
then I fill up again when I get to my destination.

2. you need more than a sample of 1 to be statistically significant.

Yup. Just throwing an early observation out.

Yousuf Khan

 

[YKhan]

You got close to my question; Yousuf, did you reset the ECU before the
test to force readjustment of the engine timing? Even so, it could take
severla 'drive cycles' before the ECU makes the proper changes.

No, didn't reset the ECU. How do you do that?

Doing
this immediately before a long trip(did you happen to increase the tire
pressure, replace the air filter or make any other changes?), with no or
limited 'on-off' or 'warm up/cool down' engine cycles is not a
reasonable test.

No changes in tire pressure, nor changes to air filter. I'm only
interested in my highway mileage at this point, I usually discount any
extended city-style driving from the results (by discounting the
results altogether). Fortunately, for my purposes, I tend to live close
to the highway for both ends of this regular trip of mine; there's only
limited city driving to get to the highway in both cases. I use city
transit during the rest of the week, so my car is mainly used as a
highway cruiser these days.

I'm only trying to guage a ballpark figure here. Need to decide whether
it's worth my money to switchover to a higher octane gas when my
understanding up until now has always been that it wasn't going to help
me.

Also, the expensive gasoline might also have more
detergent in it or some other additive. Plus, warmer weather can result
in an engine needing a little more 'headroom' as concerns octane, such
that running low octane in winter and higher octane in summer could be
advantageuos.

Yes, these are also the sort of things I'm trying to determine. The
engine is now on the higher mileage side now, it's now over 150,000 km.
So is it possible that regular is no longer the appropriate gasoline
for it anymore? Perhaps if my engine's age is causing numerous
undetected knocks and pings, which the ECU is adjusting for constantly,
but when higher octane gas is put in, it is no longer a problem and the
ECU relaxes. Another possibility is that with the additional detergents
that Super brings, maybe it's cleaning up the engine a bit?

A better test might be to fill with mid grade, clear the ECU, and
monitor about 6 tankfuls purchased from the same pump to see if there is
an improvement equal to or better than the cost increase.

Once I find out how to clear the ECU.

Yousuf Khan

 

[Mickey]

1. the higher the octate the lower the energy content of the fuel, so all
things being equal, you get less miles per gallon with a higher octane fuel.
2. 200km is about 16 litres of fuel - how are you able to measure this
accurately?
2. you need more than a sample of 1 to be statistically significant.

To add to this. Octane has nothing to do with energy content
of gasoline. It is a measure of burning characteristics.

Only time Octane will have an impact on millage is if engine
is designed to need it, timing and higher compression. You
can't do anything about compression so timing is all that is
left and computer control take care of that for the most
part. For the Subie with computer control and no dist, you
don't even have control of that.

Mickey

 

[Josh Assing]

Remember higher octane = less milage if you're computer can't adjsut for it.
the higher the octane the slower it burns....

No, I understand that, and I'm not saying this is a scientific test. I
kept my driving "style" the same on this trip. I can't account for
slight variations in traffic, temperature, etc. But I can account for
throttle and gear shifts to a certain extent, because I usually set the
cruise control once at speed, and keep it on cruise control for as long
as possible. I've learned that the cruise control is my best friend,
not only to relieve fatigue in my legs, but also I get better gas
mileage with it, because I don't tend to go up and down the speed
range, I stay steady-state on a single speed for hours on end; also I
tend to have a better average speed when cruise control is on, again
because I'm not going up and down the speed range.


Yes, but it's pretty consistently around 28 mpg when I'm cruising at
125 km/h, the variations would tend to be less than 0.5 mpg at that
speed. In the past, I've also experimented with higher and lower
cruising speeds. Whenever I've cruised upto 130 km/h, I would lose
about 1-2 mpg. Whenever I've cruised down at 120 km/h I might be
getting closer to 29 mpg. Going all of the way down to 115 km/h, I
would then tend to get over 30 mpg (usually 31 mpg). But I don't have a
lot of patience for 115 km/h, so my usual speed is 125 km/h.

So actually I got close to the same mileage I would tend to get with
regular at 115 km/h but using the higher octane doing 125 km/h.


Yes, but my understanding is that it can adjust things to optimum at 87
octane, but it can't adjust to optimum for higher octanes, it just
keeps it at the same setting as it sets for 87 octane. From sources
I've read, they say that a car designed for 87 octane won't readjust
itself for higher octanes; there are no engine mappings inside the
computer beyond 87 octane. However, cars designed for the higher
octanes can readjust themselves downward for lower octanes.

Yousuf Khan

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[Carl 1 Lucky Texan]

No, didn't reset the ECU. How do you do that?




No changes in tire pressure, nor changes to air filter. I'm only
interested in my highway mileage at this point, I usually discount any
extended city-style driving from the results (by discounting the
results altogether). Fortunately, for my purposes, I tend to live close
to the highway for both ends of this regular trip of mine; there's only
limited city driving to get to the highway in both cases. I use city
transit during the rest of the week, so my car is mainly used as a
highway cruiser these days.

I'm only trying to guage a ballpark figure here. Need to decide whether
it's worth my money to switchover to a higher octane gas when my
understanding up until now has always been that it wasn't going to help
me.




Yes, these are also the sort of things I'm trying to determine. The
engine is now on the higher mileage side now, it's now over 150,000 km.
So is it possible that regular is no longer the appropriate gasoline
for it anymore? Perhaps if my engine's age is causing numerous
undetected knocks and pings, which the ECU is adjusting for constantly,
but when higher octane gas is put in, it is no longer a problem and the
ECU relaxes. Another possibility is that with the additional detergents
that Super brings, maybe it's cleaning up the engine a bit?




Once I find out how to clear the ECU.

Yousuf Khan

Unless you have scanner capable of resetting the ECU, the 'home garage
method' is to disconnect the neg. side of the batt., tap the brake
pedal, wait - say - 20-30 minutes, reconnect the batt.(you may need to
lock/unlock with the security remote to stop lights flashing - oh, write
down your radio station presets too before this) The ECU should return
to a 'base map' and start re-learning any changes to the air/fuel/timing
settings from sensor info. It may take several 'drive cycles' to fully
adjust.

An older engine could possibly have carbon deposits built up in the
heads that effectively raise the compression. That MIGHT explain the
ability to make timing adjustments and experience a benefit from high
octane fuel. I dunno

interesting

Carl

 

[Rick Courtright]

So why would this be the case? I've heard other people in other cars
talk about this too.

Hi,

First thought is since you're driving in km/hr and getting fuel economy
in mpg, Imp g's at that, your poor computer is just simply confused. And
they think WE'RE crazy south of the 49th with OUR measuring systems!

But when the last cup of coffee of the day kicks in, it will reveal that
has nothing do do with it, and you may actually have benefited from any
number of things. Was this trip during some of the recent heat wave?
Higher temps = thinner air for less wind resistance. They also cause the
tires to run warmer, effectively raising the pressure. Higher pressure
usually = better mpg. Maybe there was a change in wind. Maybe, since I
once experienced a similar thing (long trip, small town gas station, out
of both low and med grades, had to fill up on the 'spensive juice) when
my Loyale, that was very consistent at 27-29 mpg on similar trips, and
only rarely broke 30 with a light foot and careful adherence to a
slightly lower speed than usual, turned in 38 mpg despite no respect for
a particular speed, we were fully loaded, and a coupla other "mileage
killer" factors were in play. I wrote it off as a total fluke, cuz it
never did anything like that again. Later experiments w/ higher grade
fuel showed it to be a false economy for that car. As the others have
said, one trip doesn't tell enough of the story to rely on.

Rick

 

[Chicobiker]

To add to this discussion, I recently tried a few tanks of mid-grade to
see if it would make a difference.

I ran my 03 OBS to the warning light and filled it up with mid-grade
(89 Octane?). I ran three tanks like that and although I had one
really good tank, the other two were no different than usual so I have
switched back. With mostly highway driving, I get just over
9litres/100kms.

I can't say I noticed any difference in how the car performed.


I did notice however on at least two or three trips into the US that I
got significantly better mileage on american gas. This happened in
both my Subie and my old '95 Accord. I have heard of this before, but
does anybody know what it is about american gas that allows this?

Chico

 

[Florian Feuser]

<snip>
I did notice however on at least two or three trips into the US that I
got significantly better mileage on american gas. This happened in
both my Subie and my old '95 Accord. I have heard of this before, but
does anybody know what it is about american gas that allows this?

A mesurement issue perhaps - Imperial vs. US Gallon?

florian

 

[Graeme Willox]

A mesurement issue perhaps - Imperial vs. US Gallon?

florian

A US gallon is just under 3.8 litres. An imperial gallon is just over
4.5 litres. Therefore, with all other factors being equal, you would
expect to get fewer "miles per gallon" using US gallons than imperial,
so it must be something else.

 

[Florian Feuser]

A US gallon is just under 3.8 litres. An imperial gallon is just over
4.5 litres. Therefore, with all other factors being equal, you would
expect to get fewer "miles per gallon" using US gallons than imperial,
so it must be something else.

I figured you'd fill up in CA after driving a tank of US gas and vice versa...

florian

 

[Cam Penner]

I figured you'd fill up in CA after driving a tank of US gas and vice versa...

More likely, it's a comparison of stop and go traffic at
home with longer distance highway driving in the US.

 

[Rick Courtright]

both my Subie and my old '95 Accord. I have heard of this before, but
does anybody know what it is about american gas that allows this?

Hi,

Assuming these measurements reflect your Canadian miles on Imp gallons,
and your US miles on US gallons (not hopping back and forth driving on
one side and filling up on the other!), the only thing I could think of
besides a different driving style (city vs hwy as someone suggested), is
that the actual blend of the gasoline is different. If you have a higher
"oxygenating" agent ratio on the Canadian side, it could reduce your
mileage, particularly if ethanol's the agent being used. I can't say for
sure, but I don't think we have over 10% in any of the non-E85 blends
here. The more ethanol, the lower the mileage. Also, if you guys do the
"summer-winter blend" switch like we do on this side, it may be you
crossed the border in time to get the "better" US blend before the
Canadian side changed?

And then, it could just be a fluke! Here in SoCal, people used to report
they got much better mileage on Arizona gas when they went to Phoenix
and other cities mid-state or so rather frequently. We don't hear that
story so often any more, though.

Rick

 

[Clifford Heath]

My car is an 2000 OBW,
So anyways, I decided to try out some super octane the other day.

We did a trip of 3000km recently, over six days, in our '99 Liberty RX.
Similar weather each way, and I used our regular ULP (~92 octane) on the
way up, premium (98) on the way back. Sitting on cruise control on a big
divided highway almost all the way (Hume Highway from Melbourne to northern
New South Wales).

We paid about 6% more for Premium, and got nearly 5% better consumption.
So based purely on consumption, it's not worth it, but given that the
throttle response around town is so much better on Premium, I plan to
keep buying it - it's only 1% more/distance anyhow.

Things may be different with engines made to run on 87 octane.

Clifford Heath.