What factors in cost differences between using regular vs premium fuels on our E39s?

What else might factor in cost difference calculations between using regular vs premium fuels on our E39s?

Discussion thread starting point:
Just now, in post #9 of this thread:
- 87 octane fuel OK for '02 525iT? Any negative consequences?
I calculated the cost difference as $1,200 per hundred-thousand miles (i.e., $1,200 over 5,000 gallons).

However, the 'only' factors I used for that total-cost calculation were:

• Typical ownership (defined as 100K miles)
• Typical miles per gallon (defined as 20 mpg on all three fuels)
• Typical cost difference between each stage
  • Regular to Plus = 12¢ (i.e., 0.6¢ per mile)
  • Regular to V-Power = 24¢ (i.e., 1.2¢ per mile)

Given that, the math works out to a total cost difference (per hundred-thousand miles) of:

• Regular vs Plus = $600
• Regular vs V-Power = $1,200

Note: I don't know remember the octane rating of "Plus" or "V-Power" (I assume it's 91 AKI & 93 AKI respectively)???
EDIT: As per Fudman below, I'll assume "Plus" is 89 AKI which makes the V-Power (91 AKI) the comparison point.

Out of necessity, in that post, I omitted many other factors which come into play.

This thread is intended to be a discussion of those other factors, which I hope we can order in importance, such as:

1. Potential wear & tear differences (in dollar value)
2. Projected performance differences (in calculable value)
3. Opportunity cost evaluation (e.g., if you invested that $1,200 wisely)
4. ??? what else ???

In summary:
What other factors should we consider when deciding true cost differences between using 87 AKI versus 91 or 93 AKI fuels?

Photo is of a typical gas station in the Silicon Valley a few weeks ago where I snapped this picture while waiting in line.

I believe at most gas stations, Plus is 89 AKI, not 91. At Sunoco you can get 87, 89, 91 and 93 AKI. BMW recommends 91 and I typically run 93.

During the last runup in gas price (summer 2008), on those days when I felt cheap (or exploited), I would add a half tank of 89 (when the balance was 93) to get a 91 blend. I did not notice any difference in performance between 91 and 93. On those days when I felt especially cheap and exploited, I would add a full tank of 89. There was a slight perceivable reduction in engine performance, but there was no objective evidence to verify any difference. Gas mileage was noticeably lower at 89, about 1-2 mpg (I keep records of miles traveled on every fillup), compared to 93. Since the 1-2 mpg is about a 5% reduction in mileage and the cost differential is only ~3% (assuming $4/gal), this really saved me nothing. Hence, I've decided to stay with premium (93) this time around.

My understanding is that the modern engines with onboard computers will adjust your engine to the octane used and prevent detonation. Most articles indicate that you can reduce the octane with minor reductions in performance and gas mileage without damaging your engine.

However, you need to ask yourself, is it all worth it? At the 12 cent per gallon delta, that's a mere $1.20 difference at a 10 gallon fillup. While you might save some money, those that bought a 3.0 I6 could have saved more money by buying a 2.5 I6. Or a Honda. If you ran 87 octane, your 3.0 performance might be closer to a 2.5. Since the cost of maintaining these e39s is not exactly inexpensive, gas price is just part of the "cost of doing business". And if you really want to save, get a credit car that rebates you 2-3% on every purchase. That'll give you premium gas at the cost of Plus.

Originally Posted by Fudman

Plus is 89 AKI, not 91

Thanks. I fixed that in an edit attributed to you as I wasn't sure.

That means we're looking at a potential 'opportunity cost' of about $1,200 per hundred-thousand miles as our comparison number instead of the $600 value.

If any number can withstand the facts of the discussion, then the question becomes what 'else' can you do with that amount of dollars to benefit your enjoyment of your vehicle.

Assuming there is a difference, if, for example, you use that dollar difference every hundred-thousand miles to buy new VANOS seals, a new DISA valve, replace the VCG, overhaul the CCV, cooling system & belt-drive system, new plugs and sensors, etc., then that 'may' be a better use of your money than burning it out the tailpipe.

EDIT: I saw, but don't really understand your argument that it's almost a wash ... see below ... so I think we need to better understand the tradeoffs.

Originally Posted by Fudman

Since the 1-2 mpg is about a 5% reduction in mileage and the cost differential is only ~3%

What causes that (reputed) loss in gas mileage?

In the case of our 10% ethanol, I believe the loss in gas mileage is due to the 'energy content' (right?); but the energy content of 87 AKI fuel is the same (AFAIK) as the energy content of 91 AKI fuel.

The 'only' thing different (for this discussion we're ignoring detergent blends) is that the timing 'may' be retarded momentarily under 'some' conditions.

Assuming sedate driving, when exactly is ignition/valve timing retarded?

And, if momentarily retarded, since the flame front is again proceeding smoothly, is that 'really' going to affect gas mileage in a large way?

It's easy to understand the costs of using 91 AKI over 87 AKI; but we need to better quantify the monetary costs of using 87 AKI over 91 AKI in order to come up with the true cost differential.

Doesn't matter, owner's manual calls out to use premium gas.

I also tried 93AKI sold by Husky gas stations. Usually I use Shell V-Power.
The 93 from Husky had also a reduced mpg, and I could feel also reduced performance compared to 91 AKI Shell V-Power. I believe this was due to the fact that that particular gasoline is a blend that contains ethanol. Same ethanol blend could be found at Mohawk gas stations here in Canada. No good for my car.
Best performance & fuel economy I get from Shell V-Power. So I stick with it.

Originally Posted by bluebee

...

Assuming sedate driving, when exactly is ignition/valve timing retarded?

And, if momentarily retarded, since the flame front is again proceeding smoothly, is that 'really' going to affect gas mileage in a large way?
...

Ignition timing is pretty much always adjusted by continuously detecting knocking. If BMW states that AKI 91 is optimum, then the ECU can't advance ignition more to take advantage of higher AKI-rated fuels like 93. Btw., AKI 91 is RON 95 which is pretty much the 'standard' gasoline rating in Europe. AKI 93-94 = RON 98 is used mostly by high(er)-performance cars.

If you're driving on <91 AKI, then ignition timing will only be advanced enough to the knock limit for that fuel, which is less advanced than for 91. Less advanced ignition means lower efficiency and higher fuel consumption. The difference between 87-91 AKI equates to nearly nothing in $/mile. The only difference is that the available power is higher, when needed, with AKI 91.

Does anyone know if 87 vs. 91 vs. 93 have less ethanol for stations that have blends? Seems like everywhere I go it's ethanol blend now.

Ethanol has a high octane number, something over 100, and more energy per gallon than gas.

Originally Posted by franka

Ethanol has a high octane number, something over 100, and more energy per gallon than gas.

Only half correct. Ethanol and E85 have lower specific energy content than E10 or pure gasoline. On the other hand, it has a higher resistance to knock (higher AKI) and can be used in an engine with higher compression (directly related to thermal efficiency) in order to achieve better fuel efficiency (lower fuel consumption).

Originally Posted by granlund

The difference between 87-91 AKI equates to nearly nothing in $/mile.

If you 'really' think that $1,200 is "nearly nothing", then please PM me $1,200 each time you hit one hundred thousand miles and I'll have proven my point.

Twelve hundred dollars every seven years isn't even close to 'nothing'.

So I must beg to differ. You can do a 'lot' with $1,200 every seven years in parts for your bimmer.

But, the question isn't about the $1,200 ... which is easy to calculate (& validate).

The question is what's the (real) monetary impact of using 87 AKI instead of 91 AKI fuel?

Originally Posted by granlund

Less advanced ignition means lower efficiency and higher fuel consumption

This is the statement that needs to be fleshed out for the E39.

Let's assume the statement is true for the E39. That is, if the ignition timing is retarded (& presumably the valve timing), then there is loss in miles per gallon.

The question at hand is 'how much' of a loss in miles per gallon is there with 87 AKI fuel.

Fudman, for example, estimates it's about 5%; but we need something better than a single estimate. We need facts.

Assuming we're driving around town, buying groceries, dropping the kids off at school, running errands, etc., I wonder 'if' any reduction in timing is even kicking in. Likewise, driving at a steady 65 mph on the highway, I wonder 'if' the timing reduction is even kicking in.

I suspect it might not be (but I don't really know).

The aching question is what is the true cost of using 87 AKI over 91 AKI in the E39.

It's not a philosophical question. It's a technical question.

Do we really know 'what' the difference is in miles per gallon with 89 AKI fuel versus 91 AKI fuel under typical drive-around town conditions?

Personally I suspect the difference, in 'normal' driving, is close to zero ... but that's simply my intuition. We need concrete data ... or we're all making monetary decisions based on nothing.

Factually, do we have any 'real' evidence of a MPG difference in the E39 when using 89 over 91 AKI?

I'm not sure why you don't try it and find out. We have done this on several of our cars. We calculate the average mpg over 2-3 tanks of one octane, then the next, then the third, and figure out, if any mpg difference, how much difference and if it is worth it to pay more for the higher octane. For instance, on my wife's town & country, we've found no difference in octanes, so she runs regular now. On her previous XC90, however, we found it was worth it to run 93. You aren't going to hurt the car, so go ahead and experiment.

Originally Posted by bluebee

If you 'really' think that $1,200 is "nearly nothing", then please PM me $1,200 each time you hit one hundred thousand miles and I'll have proven my point.
...

My point was to say that the difference in cost for AKI, offset by the difference in fuel consumption, equates to that the $/mile difference is nearly nothing.

Originally Posted by bluebee

...
Let's assume the statement is true for the E39. That is, if the ignition timing is retarded (& presumably the valve timing), then there is loss in miles per gallon.
...
Assuming we're driving around town, buying groceries, dropping the kids off at school, running errands, etc., I wonder 'if' any reduction in timing is even kicking in. Likewise, driving at a steady 65 mph on the highway, I wonder 'if' the timing reduction is even kicking in.
...

The ignition is continually advanced to the knock limit when you drive. Based on the rpm, throttle position and intake air temperature (and probably a few other parameters as well), there is a preset value for ignition advance. Continuously while driving, this is advanced further by the ECU until knock is detected and then retarded slightly. Repeat advance-retard-advance-retard continuously to be at the knock limit.

For lower AKI fuel, the mean advance position is less than for higher AKI.

Originally Posted by bluebee

The aching question is what is the true cost of using 87 AKI over 91 AKI in the E39.

It's not a philosophical question. It's a technical question.

Do we really know 'what' the difference is in miles per gallon with 89 AKI fuel versus 91 AKI fuel under typical drive-around town conditions?

BB:
The answer you are seeking cannot be determined quantitatively or accurately unless a carefully controlled and executed experiment is conducted. As with almost everything that is discussed on this forum, the feedback you are getting will be subjective and anecdotal. Here is what Car & Driver says, based on testing (that included a BMW M3) they conducted:
http://www.caranddriver.com/features...results_page_2

"Our tests confirm that for most cars there is no compelling reason to buy more expensive fuel than the factory recommends, as any performance gain realized will surely be far less than the percentage hike in price. Cheapskates burning regular in cars designed to run on premium fuel can expect to trim performance by about the same percent they save at the pump. If the car is sufficiently new and sophisticated, it may not suffer any ill effects, but all such skinflints should be ready to switch back to premium at the first sign of knock or other drivability woes."

This article suggests that since BMW recommends 91, there is no advantage to buying 93. But running 89 will have a corresponding reduction in performance. This is why I said using regular gas in an e39 is equivalent of having a lower performing engine. Not a great analogy but why spend the money on a performance car if you don't optimize the performance? They make no mention of gas mileage in this article. My observed reduction in mpg may be attributed to unconsciencely stepping on the gas harder or it might be due to other factors. Who knows...

Originally Posted by Qbrozen

I'm not sure why you don't try it and find out

Good point!

As suggested, I 'will' try to compare 'my' MPG with both Costco 87 AKI & 91 AKI on the next few set of refills - but - again - truth be told, I doubt it will be possible to conclude anything meaningful because the stated differences (5%) are wholly within the known errors in our measurements (4% to 5%).

See this thread for full details:
- What is the tolerance (i.e., accuracy) of our typical miles per gallon (MPG) calculations (1)

Originally Posted by Qbrozen

calculate the average mpg over 2-3 tanks of one octane, then the next, then the third, and figure out, if any mpg difference

That is a good method because it averages out 'some' of the factors (e.g., how high you fill the tank, the temperature of the fuel when you filled up, the weather, the traffic, your driving style, the length of the trips, the load being carried, etc.).

But note that even the EPA, on a 5-cycle known test run, with professional drivers, on a known circuit with a precise set of controlled driving circumstances, still gets a 4% accuracy scattering.

The problem is that, if Fudman is right about the 5% mpg difference between 87 AKI and 91 AKI, you'd never notice it in the calculations because of the 4% to 5% error in his calculations from the start. Right?

BTW, it may be instructive to note that I 'did' test 87 AKI versus 91 AKI for the California emissions testing regimen, where, predictably, we found no statistical difference between the emissions results between the two fuels.

In fact, the lower-octane fuel resulted in (very) slightly better emissions results than did the 91 AKI fuel (however the California smog tests were done two weeks apart, using the same vehicle, and the same testing station, with absolutely no changes to the vehicle in the interim).
- Comparing California emissions results between 87 AKI and 91 AKI test fuels (1)

Originally Posted by granlund

My point was to say that the difference in cost for AKI, offset by the difference in fuel consumption, equates to that the $/mile difference is nearly nothing.

I understand that point. In fact, it's the entire point of this thread. The problem is quantifying that point.

If, for example, you save $1,200 by using 87 AKI fuel, yet, the MPG difference equates to, say, a loss of $1,000, then you only 'saved' $200 in reality - which - for most of us - wouldn't give us much by way of opportunity.

On the other hand, if, for example, you save $1,200 by using 87 AKI fuel, yet, the MPG difference equates to $0 or, say, $200, then you 'do' save a real $1,000 every hundred-thousand miles (anyone who thinks a thousand dollars is "nothing" would do well to consider what their car could do with $1,000 worth of new parts).

We're both saying the same thing ... (you're stating it as if it's a fact ... I'm simply asking for believable proof).

None of us can conclude anything meaningful costwise unless we determine:

1. What is the monetary negative MPG impact of using 87 AKI versus 91 AKI
   • So far, the reputed figures quoted (5%) appear to be within the error in our calculations (4% to 5%).
2. What is the monetary engine-repair impact of using 87 AKI versus 91 AKI
   • So far, zero negative monetary repair-cost impact has been proposed.
   • Note: Any reputed 'performance' impact isn't meaningful for this discussion unless there is a quantifiable monetary association.
3. Are there any other purported quantifiable cost impacts?

Fact is, all the statements here are either true or false depending on whether there really is a negative cost difference - and - if there is, how large it is.

Originally Posted by Qbrozen

The ignition is continually advanced to the knock limit when you drive.

I'm glad you said this as it gives you credibility because you understand that the most efficient fuel is one that is 'almost' knocking.

Originally Posted by Qbrozen

For lower AKI fuel, the mean advance position is less than for higher AKI.

Interesting. You explained that well. So, what you're saying is that my timing (with 87 AKI) is certainly retarded from yours (at 91 AKI) because of the inherent knock tendencies of our respective fuels - even if I do drive sedately.

If so, that begs a question. We'd need to figure out how many degrees that additional retardation is, and whether those degrees are themselves meaningful. For example, if it's 1° BTDC different ... is that meaningful (in terms of mpg)?

QUESTION: What is the total possible 'range' of degrees BTDC retardation that the ECU can effect?

Originally Posted by Fudman

The answer you are seeking cannot be determined quantitatively or accurately unless a carefully controlled and executed experiment is conducted.

Precisely! I seek data. Real data. Believable data. Quantifiable data.

Originally Posted by Fudman

Here is what Car & Driver says ...

I wholly agree with what you excerpted.

Originally Posted by Fudman

running 89 will have a corresponding reduction in performance.

I don't disagree (other than to say the performance is 'just fine' on 87 AKI with the way I drive sedately). However, 'performance' isn't the question here. It's 'cost' we're trying to figure out. Specifically, either MPG cost or engine-repair cost (or any other quantifiable cost).

Originally Posted by Fudman

This is why I said using regular gas in an e39 is equivalent of having a lower performing engine.

I don't disagree.

I certainly understand the desire for maximum attainable performance. But, to be blunt, performance isn't the question. Nobody has suggested performance is better on 87 AKI than on 91 AKI.

What we're looking for here are 'costs' (in dollars). Of course, you can arbitrarily put a dollar value on performance - but that's a different question altogether - for a different thread.

This thread is all about what the costs are (if any) to using 87 AKI over 91 AKI fuel (the savings are easy to quantify).

Obvious saving minus imputed costs = the true savings (if any).

Originally Posted by Fudman

why spend the money on a performance car if you don't optimize the performance?

That's a wonderful philosophical question. But for a different thread.

This thread questions what the cost saving really is for using 87 AKI over 91 AKI. This cost saving can be as high as $1,200 every hundred thousand miles - or the cost saving can be reduced by either the reputed loss in MPG or in purported damage to the engine.

As in so many things here on Bimmerfest, once we question the oft-held assumptions, we 'may' find that they are based (meaninglessly) on warm-fuzzy emotion more so than cold hard facts.

Originally Posted by Fudman

They make no mention of gas mileage in this article.

Indeed! I suspect there is no measurable difference in MPG, despite the common belief that there is ... but that's just my (unproven) hypothesis.

All I can say is that our typical MPG calculations are off by 4% or 5% from the start - so any quoted MPG difference not greater than that is (statistically) in the noise level.

Originally Posted by Fudman

My observed reduction in mpg may be attributed to unconsciously stepping on the gas harder or it might be due to other factors. Who knows...

Exactly!

Who knows?

The question, (perhaps to remain unanswered), is whether there are 'any' truly meaningful negative monetary costs to using 87 AKI over 91 AKI fuel.

This is one potential conclusion (based on the data, so far):

• (price savings) - (mpg losses) - (engine damage) = true savings
  • Price saving = $1,200 every 100K miles
  • MPG losses = 5% stated mpg loss +/- 4% to 5% imputed error
  • engine damage = $0 stated (so far)

Note: Here are my spark plugs on my M54 at 100K miles after about 70K miles on 87 AKI fuel:

I've owned my 2000 540i since 2004 when it had 90K miles. I'm now at 241K and I've mostly used regular gas throught the life of the car. A few times I've felt guilty and gotten her mid grade or high grade. I get about 22mpg combined driving. My wife's X3 3.0 6M gets about 24mpg and she uses regular as well. 140K on that car. No engine or tranny issues with either car yet, knock on wood. My non proven opinion is, if you don't drive them like race cars, you can get away with regular. I also wonder when our cars get up in mileage if they develop slighter lower compression, which allows them to use regular as well.

Originally Posted by franka

Ethanol has a high octane number, something over 100, and more energy per gallon than gas.

Frank, I don't doubt it.
But here's another story: A few years back I fueled my F150 with ethanol blended gasoline. (regular or AKI 87).
When I was on the Hwy, I had to pass someone. I was driving around 100 Km/Hr (about 65 mph). I pressed the gas pedal, and...nothing. It took my truck forever to reach 120 Km/Hr (75 mph) in order to pass. I also noticed that it was quasi impossible to reach the governed speed limit, which was quite low on that truck. First I thought that something went wrong with the trucks engine. Asked a few guys atound and one actually asked if used ethanol blended gasoline. That was the culprit. Once I refilled the tank with "normal" gasoline, the truck was as usual - not peppy, but I could pass on the Hwy!!!!
So if the ethanol is great, why does it impeach performance?
When I tried the 93 (or was it 94 AKI ethanol blended?) on my BMW, I thought maybe it was bad on the Triton engine...not. Same happened on the M54.
I will not use ethanol blended gasoline ever, unless I have no choice...

Unfortunatly I don;t have a choice here and I MUST use ethanol blended gas.
There are no gas stations within 45 minute drive that have pure gas.

My choices are 87, 89 and 93. Only 1 gas station I know of has 91.

I stick with Sunoco, Shell, Chevron and Mobil gas. 93 octane.
I haven't tried a lower grade, but honestly i'm not really wanting to. The car runs silky freaking smoothe and has pep to it.
The only thing I see a lower octane rating gas doing for the car is causing more deposits to built up, less you use something like Chevron Techron more often.
However, if you are using an additive more often, then that defeats the purpose of what you would save at the pump, would it not?

Manufacturers don't arbitrarily put a recommended octane rating on the car just for kicks - there's rhyme and reason to why they do it and I don't want to chance screwing something up just to save a few bucks.
Case in point - I had a Mazda 6 and regular was recommended. I tested out 93 for a few weeks and the car got poor gas mileage and not much better performance. Regular was the way to go on that car. Other Mazda 6 owners reported the same findings.

bluebee - i'm wondering if you should factor in an 'additive' (ex: Chevron Techron) cost for 87 vs 91. I would assume that more additive would need to be used per 100k miles if you were using 87 vs 91. Thoughts?

Lots of pointless info. Just put the 91 and you are set. Does not matter from what gas station. Shell and other small gas stations all use the same gas. Shell just adds a few things and calls it V-Power. Ohhhhh...... V-Power. Lets put that in. Ferrari uses it.

So while working on a friends e39 today this post came up in conversation


The cost of $1200 over 100000 miles is $0.01 cents per mile really not worth the time spent thinking about it but since we were already kicking back some cold drinks and knocking out a rear shock replacement i got to thinking. . .

If we assume your e39 gets 26mpg on average and you parked it at your local Ford dealer and bought a brand new Fiesta for retail with no trade in ($13,995) you could drive. . .


100,000miles in your BMW = 3,862 gallons of premium fuel per BMWs recommendations, which comes to $17,263 @ $4.47


100,000miles = 2500 gallons of standard fuel in the fiesta(@40mpg) which comes to $10,575 in fuel @ an average cost per gallon of $4.23


What you'll find is that over 200k the fiesta will nearly pay for itself in fuel savings! Awesome!

But I greatly prefer my BMW and will continue to drive it even if the "cost" doesn't make it the most economical choice in the end, perhaps you should look into a fiesta if your that concerned about .01 per mile?

BMW- .17 per mile
Fiesta- .11 per mile <- and that's before you begin to compare the maintenance cost differences . . .

Originally Posted by bimmerteck

math

Welcome to the 'fest

Originally Posted by agent15

Welcome to the 'fest

+1

... from another Tennessean!

Originally Posted by bimmerteck

So while working on a friends e39 today this post came up in conversation


The cost of $1200 over 100000 miles is $0.01 cents per mile really not worth the time spent thinking about it but since we were already kicking back some cold drinks and knocking out a rear shock replacement i got to thinking. . .

If we assume your e39 gets 26mpg on average and you parked it at your local Ford dealer and bought a brand new Fiesta for retail with no trade in ($13,995) you could drive. . .


100,000miles in your BMW = 3,862 gallons of premium fuel per BMWs recommendations, which comes to $17,263 @ $4.47


100,000miles = 2500 gallons of standard fuel in the fiesta(@40mpg) which comes to $10,575 in fuel @ an average cost per gallon of $4.23


What you'll find is that over 200k the fiesta will nearly pay for itself in fuel savings! Awesome!

But I greatly prefer my BMW and will continue to drive it even if the "cost" doesn't make it the most economical choice in the end, perhaps you should look into a fiesta if your that concerned about .01 per mile?

BMW- .17 per mile
Fiesta- .11 per mile <- and that's before you begin to compare the maintenance cost differences . . .

Very well put!

Originally Posted by repcapale

lots of pointless info. Just put the 91 and you are set. Does not matter from what gas station. Shell and other small gas stations all use the same gas. Shell just adds a few things and calls it v-power. Ohhhhh...... V-power. Lets put that in. Ferrari uses it.

+1 +1 +1

Thanks for doing the home work for me bb

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