[ages944]

I've read that fuel octane isn't supposed to affect mileage, but I've been noticing a marked improvement in fuel economy part way through my first tankful of 'mid-grade' gasoline (89 octane). I threw some in as part of my dealer's recommendation to try to alleviate a rough idle. Anyway, though I don't know if this will hold, I've been observing a fuel economy so far (after 150 mi) of about 27mpg in mixed driving. Generally, I usually get 23-24mpg overall per tankful.

Has anyone else tried mid-grade in their car, and noticed similar fuel economy? Might I have stumbled onto something here perhaps?

[Asiann]

That would be interesting to find out. Run through some more tanks and see if it is consistent.

[foofighter]

that's really interesting...definitely run more than a tank to get the ecu to adapt

[Asiann]

I would save about 10 cents per gallon. Right now we are paying $3.29:mad:

[blke90man]

Originally Posted by Asiann

I would save about 10 cents per gallon. Right now we are paying $3.29:mad:

Wow...and I thought gas was more expensive in Hawaii. I just filled up the other day: $3.64/gal

[Asiann]

Originally Posted by blke90man

Wow...and I thought gas was more expensive in Hawaii. I just filled up the other day: $3.64/gal

Depends on where you go. That is the cheapest Prem on the island. It is worse in some places as far as you are paying. There is one island that only has 1 gas station and is the worst gas price in the US but they don't mention it in the news for some reason.

[blke90man]

Originally Posted by Asiann

Depends on where you go. That is the cheapest Prem on the island. It is worse in some places as far as you are paying. There is one island that only has 1 gas station and is the worst gas price in the US but they don't mention it in the news for some reason.

Yeah I can definitely see Hawaii having the highest gas prices.

[Asiann]

Originally Posted by blke90man

Yeah I can definitely see Hawaii having the highest gas prices.

It has gotten better since they lifted the gas cap law. Prices would change weekly for a long time until that. I still cannot understand why Hawaii choose to base prices off of Cali, NY and Singapore? I know we are away from civilization but come on!:mad:

[330m]

Originally Posted by ages944

I've read that fuel octane isn't supposed to affect mileage, but I've been noticing a marked improvement in fuel economy part way through my first tankful of 'mid-grade' gasoline (89 octane). I threw some in as part of my dealer's recommendation to try to alleviate a rough idle. Anyway, though I don't know if this will hold, I've been observing a fuel economy so far (after 150 mi) of about 27mpg in mixed driving. Generally, I usually get 23-24mpg overall per tankful.

Has anyone else tried mid-grade in their car, and noticed similar fuel economy? Might I have stumbled onto something here perhaps?

Did you go up from low grade or down from Premium??

Surely you should only use premium in you pride and joy? I wouldn't use anything less.

[martieg]

Octane doesn't affect mileage at all, unless you use a lower grade that produces knocking.

The BMW E90 engine is a high compression engine, that's why a high octane number is required. High compression engines would produce knocking with a lower grade gasoline.

A major factor in mileage is the additive that the gasoline has. All gasolines are basically the same quality before they add the additive. The gasoline additive is general terms cleans the engine components from damaging deposits from the combustion.

I have experienced sustained superior performance and better mileage using a premium gasoline. My advice is don't try to save using a cheaper gasoline - buy a premium gasoline with a high octane.

For more info, the following link is excellent: http://science.howstuffworks.com/gasoline.htm/printable

[330m]

Originally Posted by lecatrache

Octane doesn't affect mileage at all, unless you use a lower grade that produces knocking.

But all grades will knock with enough timing advance, it's just that the engine management detects the knock point and controls the ignition timing to suit.

With a lower grade fuel, the ignition will be backed off, making combustion process is less efficent, so the engine needs to use more fuel (i.e. you have to put your foot down further) to obtain the same specific power output.

Therfore it does less miles to the gallon! (only marginal difference though)

[Dankim10]

Originally Posted by blke90man

Wow...and I thought gas was more expensive in Hawaii. I just filled up the other day: $3.64/gal

I pay for the octane 95, $1.50/gal. Pretty cheap huh?

[bmwhomosapien]

down here in texas..... its 2.89 for octane 91

[Krockit]

Just picked up my 330i Wednesday. During delivery the salesman (a knowledgable, good guy) said it's no problem burning lower octane grades. Only difference is the straightline performance will be slower. I questioned him on this but he was confident and adamant- this will not harm the engine. I'm still going to run premium. .15 per gal. is the diff here. That's just $2.40 a tank.

[Asiann]

Originally Posted by dankim10

I pay for the octane 95, $1.50/gal. Pretty cheap huh?

I think Iraq only pays .6 Now that is cheap!

[Asiann]

Originally Posted by Krockit

Just picked up my 330i Wednesday. During delivery the salesman (a knowledgable, good guy) said it's no problem burning lower octane grades. Only difference is the straightline performance will be slower. I questioned him on this but he was confident and adamant- this will not harm the engine. I'm still going to run premium. .15 per gal. is the diff here. That's just $2.40 a tank.

Sorry but a salesman is just that unless his previous job was teaching compostion of gasoline and its effect on gas mileage. Better to stick with the recommended gas and just pay a little more IMO.

[ages944]

Originally Posted by lecatrache

A major factor in mileage is the additive that the gasoline has. All gasolines are basically the same quality before they add the additive. The gasoline additive is general terms cleans the engine components from damaging deposits from the combustion.

The reason I went down to midgrade for this tank was precisely to put gasoline with some different additives into the engine, with the goal being to help clean out the engine a bit (again, this was done on the advice of the service foreman at my dealership). It's not a price based decision, and I don't plan on making it a regular practice, unless there's some real observed benefit from it (one tankful won't convince me).



Couple of points:
1st, I went down from premium to mid-grade. Up till now, I've never put anything but premium in the car.

2nd, 89 octane (mid grade around here) is only 2 points lower than the minimum. I don't suppose that the engine's ECU has any trouble adjusting for the difference. lecatrache hit it on the head here, in that higher octanes are mainly needed in high compression engines to avoid knock. As long as the ECU can control iginition timing to compensate, everything remains dandy.


To me, I'm theorizing that the two biggest factors improving the economy would be:
- the differences between the grades (additives and such)
- the engine ECU's ability to adjust for the the difference in fuel grade


Perhaps it still too early to tell. After a week and 150mi, I've still got about 2/3 of a tank remaining. I'll finish out the tank and make a decision for what to do next at that time.

As an anecdote (and OT from this a bit), the service foreman was telling me that the V8's in the 5's and 7's seem to hate premium... weird.

[S62FAN]

It averages 27 mpg @ 40mph average speed. Never had any ill effects, and I didn't notice any difference using 93 octane. Why throw money away?

[RGRAY]

Originally Posted by 330m

...Surely you should only use premium in you pride and joy? I wouldn't use anything less.

I agree totally.

[Asiann]

Originally Posted by S62FAN

It averages 27 mpg @ 40mph average speed. Never had any ill effects, and I didn't notice any difference using 93 octane. Why throw money away?

"Well according to the manual, The miniumum octae rating is 91. If you use gasoline with this minimumu octane rating, the engine may produce knocking sounds when starting at high outside temperatures. The use of poor-quality fuels may result in drivability, starting and stalling problems especially during certain environmental conditions such as high ambient temperature and high altitude. Should you encounter drivability problems which you suspect could be releated to the fuel you are using, we recommend that you respond by switching to a recognized high-quality brand. Failure to comply with these recommendations may also result in unscheduled matintenance." Page 107 in the owners manual. Prevention goes a long ways!