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Not a real doctor.
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Discussion Starter · #1 ·
I've taken advantage of a rainy day to compile more data about my January build coupe.

First, mileage:



I attribute the general, modest, upward trend through June to the mild increases in average temperature during that period.

From mid-August to date, I've been trying for higher mileage by having fun during accelerations, but then using the biggest gear possible to keep the revs below 2k. I refer to this as "driving in Prius mode." Not as much fun while driving, but I enjoy the challenge of seeing if I can break 30 mpg. (Not quite yet.)

Next, how accurate is the OBC fuel consumption gage?



I really can't explain why it's been getting better, but as you can see, my OBC mileage is stabilizing at only about 1% to 2% optimistic. Pretty accurate, really. I'm impressed.

Next, fuel prices.



With only one exception, a tankfull of Sunoco premium 93 octane, I have only filled with Shell premium gasoline which, around here, is always 93 octane. I wonder how long we'll have $3/gallon gasoline.

The last time I posted, somebody asked how linear our fuel gages were. I started keeping track. Here are the results:



For those math and science types, look at that R value! But you don't need a college degree to see how linear these results are.

Two important take away points here:
1. When my fuel gage reads zero, I can put 15.64 gallons in. That doesn't mean the tank is dry though... there still might be some left even when the gage reads zero.
2. When you fill up, there's about 1 gallon of gas "extra" beyond the fuel gage reading of 100%. So I should be able to drive about 20 to 30 miles before the gas gage begins to show a value below 100% full.

Finally, how does the BMW's OBC Miles to Go value measure up?



I got my own value by taking 15.64 as my total capacity, subtracting out how much I filled to get the estimated fuel remaining in the tank. I then multiplied that by my calculated fuel consumption to estimate how many miles I would have been able to go before my gage read zero.

As you can see, the BMW is consistently optimistic and from the equation of the linear fit, the BMW is optimistic by about 24%. That's important. It tells me not to put too much faith in the OBC's estimate. If the OBC tells me I have 100 miles to go, I now know I only have about 80. Now, it's possible that the OBC is taking into consideration some amount of fuel that still exists in the system when the fuel gage reads zero, but if that were true, the linear best fit shouldn't have an intercept at the origin.

Overall conclusions:
1. You can drastically improve your mileage by keeping your revs low. (Not that you want to always drive that way... gotta have some fun too right?)
2. The OBC consumption value is pretty darned accurate.
3. The fuel gage is beautifully linear but there's an extra gallon hiding beyond "full."
4. The miles to go value from the OBC appears to be optimistic by more than 20%. Careful!
5. Who the hell knows where fuel prices are going to be a month from now?
 

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Not a real doctor.
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Discussion Starter · #4 ·
Thanks, FLA.

Hey, Saintor, I don't know anything about hidden menus. I have no iDrive. Do I still have hidden menus?
 

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Here comes trouble!
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Not a real doctor.
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Discussion Starter · #6 ·
Hmmm... interesting. 'Not sure I'd mess with this in my car, but I'm still curious as to whether or not it exists in the E90 series.
 

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Here comes trouble!
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Hmmm... interesting. 'Not sure I'd mess with this in my car, but I'm still curious as to whether or not it exists in the E90 series.
Exactly my thoughts.

Whoever get's a dealership loaner E9X first tries it, k?

:D
 

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I'd rather be driving!
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Nice break down Professor, good info to know! Thanks! :D
 

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Not a real doctor.
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Discussion Starter · #10 ·
A pleasure, guys. No problem.

No comments about driving a 328i in "Prius mode?"

(I open myself up for abuse like this, I expect takers.)
 

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Here comes trouble!
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Prof, when you start a thread, your usual abusers stay out. See DSX anywhere? Nope. Xrated? Nope. Why? Because they know what to expect and you did not disappointed. 5 graphs, multiple numbered lists, and a conclusion. They never got to the "Prius Mode" part.

Here's my best shot: Driving a 328 in Prius Mode is nothing but a rebadged....ah heck, who am I kidding. :dunno:
 

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Tastes like chicken
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Great job prof!! I've been wondering many of the same things since I've had this--mainly how accurately the OBC mpg represents actual MPG. I'm actually impressed it shadows real MPG as closely as it does.

Do you take requests for research? I'd like to see you average a month of driving using nothing but D (drive as you normally would). Then run a month of driving using nothing but DS doing the same. I'm curious to see how much of a fuel consumption hit you take using DS vs D. Of course, that would be a couple of very boring months of driving!!

EDIT: Never mind that second part--I just saw you have a MT. :rolleyes:
 

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Tastes like chicken
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4. The miles to go value from the OBC appears to be optimistic by more than 20%. Careful!
Just a thought on this observation: gas up here in the NE is required to have 10% ethanol, which as some may know is less efficient than gasoline when being converted to energy (another way to think of it is a gallon of gasoline 'contains' more energy than a gallon of ethanol). I'm wondering if the OBC is calibrated with pure gasoline, and the ethanol is part of the reason for the variance. It would be interesting to see these results from someone in another part of the country where ethanol is not required.
 

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Boosted Bavarian
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Hmmm... interesting. 'Not sure I'd mess with this in my car, but I'm still curious as to whether or not it exists in the E90 series.
Come on… you know you want to!

Nice work, as usual, Professor. Now you've got me motivated. It's been awhile since I transferred the data from my log book, so and my spreadsheets are dated.
 

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Not a real doctor.
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Discussion Starter · #15 ·
Prof, when you start a thread, your usual abusers stay out. See DSX anywhere? Nope. Xrated? Nope. Why? Because they know what to expect and you did not disappointed. 5 graphs, multiple numbered lists, and a conclusion. They never got to the "Prius Mode" part.

Here's my best shot: Driving a 328 in Prius Mode is nothing but a rebadged....ah heck, who am I kidding. :dunno:
Fine points all, Winter.

I would like to point out that it's easier to drive a 328 in Prius mode than to drive a Prius in 328 mode.
 

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Not a real doctor.
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Discussion Starter · #16 ·
Just a thought on this observation: gas up here in the NE is required to have 10% ethanol, which as some may know is less efficient than gasoline when being converted to energy (another way to think of it is a gallon of gasoline 'contains' more energy than a gallon of ethanol). I'm wondering if the OBC is calibrated with pure gasoline, and the ethanol is part of the reason for the variance. It would be interesting to see these results from someone in another part of the country where ethanol is not required.
I think I read somewhere around here that the BMW uses some sort of instrument to measure the actual volume of fuel going into the engine at any one time. If so, there would be no need to compensate for an ethanol mix (which does indeed carry less potential energy per volume).

I do understand that here in the great Northeast, there is a switch to a winter blend at some point. I would expect my mileage to change when that happens even if I were to keep driving the same way.
 

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Not a real doctor.
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Discussion Starter · #17 ·
Nice work, as usual, Professor. Now you've got me motivated. It's been awhile since I transferred the data from my log book, so and my spreadsheets are dated.
Oh ho! Then let's see them!

(I don't think you should ever trust research until a number of folks come up with the same results. After all, I could've faked all those graphs up there.)
 

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detour in Ingolstadt
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This is why I trust BMW with my luxury car purchases. I never need to question the engineering and precision, and when I do I like the answers.
 

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Not a real doctor.
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Discussion Starter · #19 ·
I'm particularly pleased with the fuel gage. The thing actually is useful. 'Much better than the types that only have marks at empty, half full, and full and even then, the marks are only rough estimates. By George, I now know that when the gage reads 20%, I've got 3 gallons left in the tank.

Of course, when you get right down to it, I don't really need to know that. But somehow it appeals to the engineer in me.
 

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Fuel Consumption



With only one exception, a tankfull of Sunoco premium 93 octane, I have only filled with Shell premium gasoline which, around here, is always 93 octane
Interesting you mentioned that, Professor. I've noticed in my 2004 x3 that my OBC gives higher mpg readings with Shell 93 octane than any other brand. The difference can be drastic for similar tachometer readings. Anyone else notice something similar?
 
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