Better gas mileage with Super octane?

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 >My car is an 2000 OBW, with a 2.5L H4, recommended gasoline is just

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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?
3. 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

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

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.

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.

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

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.

Yup. Just throwing an early observation out.

Yousuf Khan

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

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

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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

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

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

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.

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

florian

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

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

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.