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Old 10-08-2017, 06:32 AM
smassey321 smassey321 is offline
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Diesel MPG at 80 MPH?

I am curious what others get at 80 MPH. I am coming up on 2 years of ownership and I have never exceeded 22 MPG. We live in FL and 80 MPH is normal for interstate travel here. The good news is we are at sea level and there are no hills. AC is on except for cold mornings and odd winter days.

Slowing to 75 may help a .5 mpg but I have never seen the 27 MPG many claim to get. Doing a MPG reset and driving at 55 will show 26 mpg while moving for many miles but that will drop back down at the first stoplight. Driving aggressively does not lower the MPG significantly.

I bought the 2009 35d in late 2015 w/ 30k miles and it now has 70k. The only thing odd was it did not get the EGR software updates until 40k miles in 2016 so it has some cbu on the intake valves. However all the intake sooting tests in ISTA+ pass.
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Old 10-08-2017, 06:58 AM
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Doug Huffman Doug Huffman is offline
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I think 27 mpg @ 80 mph is a good number.
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Old 10-08-2017, 07:01 AM
ATL Markus ATL Markus is offline
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The only times I get more than 23 mpg on the highway are when i am slower than 80mph. If I stay between 75 and 80 I can get 26 mpg. I usually drive 85+ and get 22.
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Old 10-08-2017, 07:37 AM
Autoputzer Autoputzer is offline
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Diesels like to run slow. Also, X5's are a brick, aerodynamically. The aerodynamic drag force, and the energy (and fuel) to overcome it, are roughly proportional to the square of velocity. So, going 80 MPH uses at least 14% more fuel and energy than 75 MPH ((80^2/75^2) -1), and at least 31% ((80^2/70^2)-1) more fuel than 70 MPH. Those percentages assume that the engine's efficiency remains the same as speed varies. If the engine's efficiency goes down with increase speed, the percentages would be larger.
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Old 10-08-2017, 08:33 AM
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Scepticism and Animal Faith (1923)
Scepticism is the chastity of the intellect, and it is shameful to surrender it too soon or to the first comer: there is nobility in preserving it coolly and proudly through long youth, until at last, in the ripeness of instinct and discretion, it can be safely exchanged for fidelity and happiness.
(The Works of George Santayana p. 65)

Eschew eristical argumentation. I am responsible for what I write, not for your understanding of it.
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Old 10-08-2017, 11:15 AM
edycol edycol is offline
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This is not scientific but maybe it can help you.
My house is at 6800ft. Driving from Colorado Springs to Denver that is at 5260ft at 80mph without hard acceleration etc will give me 28-30mpg. Going back I gain in altitude so if same driving style I average 26-27mpg. That is unusual since I do drive aggressively and often exceed 100mph passing cars. So without kid and wife I usually get 25mpg avg. Now going to ski I average around 24mpg. But if I put ski rack on the roof it lowers mpg to 21-22mpg.
My question is:
1. Is your thermostat ok?
2. What oil you use?
3. Alignment ok?
4. What kind of tires you have?
5. They properly inflated?

Although I drive aggressively and Colorado Springs is not as urban as San Francisco or NYC, I get in city 21-22mpg. I do use gears to slow car.


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Old 10-08-2017, 03:44 PM
smassey321 smassey321 is offline
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Quote:
Originally Posted by edycol View Post
This is not scientific but maybe it can help you.
My house is at 6800ft. Driving from Colorado Springs to Denver that is at 5260ft at 80mph without hard acceleration etc will give me 28-30mpg. Going back I gain in altitude so if same driving style I average 26-27mpg. That is unusual since I do drive aggressively and often exceed 100mph passing cars. So without kid and wife I usually get 25mpg avg. Now going to ski I average around 24mpg. But if I put ski rack on the roof it lowers mpg to 21-22mpg.
My question is:
1. Is your thermostat ok?
2. What oil you use?
3. Alignment ok?
4. What kind of tires you have?
5. They properly inflated?

Although I drive aggressively and Colorado Springs is not as urban as San Francisco or NYC, I get in city 21-22mpg. I do use gears to slow car.


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Thermostat was replaced last year.
I use BMW oil
Alignment was done last year
Tires are 19" Michelin Latitude Tour but are not run flat.
Tires are at stock inflation.

Also, my city and highway MPG is always 22 MPG. City driving does not drop it down.
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Old 10-08-2017, 03:52 PM
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Quote:
Originally Posted by smassey321 View Post
Also, my city and highway MPG is always 22 MPG. City driving does not drop it down.
something’s not right.

I can take off a half a mile per gallon just passing interstate traffic. A long hill will take 2/10 off and I can get them back drafting a semi.
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Scepticism and Animal Faith (1923)
Scepticism is the chastity of the intellect, and it is shameful to surrender it too soon or to the first comer: there is nobility in preserving it coolly and proudly through long youth, until at last, in the ripeness of instinct and discretion, it can be safely exchanged for fidelity and happiness.
(The Works of George Santayana p. 65)

Eschew eristical argumentation. I am responsible for what I write, not for your understanding of it.
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Old 10-08-2017, 04:08 PM
francispj francispj is offline
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I assume it's an X5 not an E90 ( unclear from comments)
If a car that would be too high but maybe X5 would be ok

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Old 10-08-2017, 04:26 PM
edycol edycol is offline
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Quote:
Originally Posted by smassey321 View Post
Thermostat was replaced last year.

I use BMW oil

Alignment was done last year

Tires are 19" Michelin Latitude Tour but are not run flat.

Tires are at stock inflation.



Also, my city and highway MPG is always 22 MPG. City driving does not drop it down.


Something doesn’t add up there. If your city mpg is 22mpg then that indicates that everything works perfectly. If you cannot go over 25mpg on interstate, then that indicates that something is wrong.
My mpg is bit high because in Colorado air is less dense, so generally cars get better mpg. Do you reset your trip computer every time you fill up tank?


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  #11  
Old 10-08-2017, 04:39 PM
Michael47 Michael47 is offline
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Being at sea level is no help, you know. Actually, it is a slight detriment, in that wind resistance will be greater at sea level than at altitude. But not by much at all, unless you make it to say 30,000 feet.
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Old 10-08-2017, 05:03 PM
FredoinSF FredoinSF is online now
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Diesel MPG at 80 MPH?

22MPG at 80 on flat terrain seems low indeed. Do you have Carly or some other diagnosis tool? You can typically monitor several parameters concurrently. Would be nice to see a speed RPM MPG line graph for the sake of objectivity.

FWIW I get roughly 24MPG for each tank. 80% of my driving is highway at 78MPH (actual, not BMW buffered value), but I am not on flat terrain so not sure how that works out as a comparison to your situation since logically mileage drops going up but then improves coming down the same hill on the reverse trip.
The remaining 20% is in town driving and I have steep hills there too.


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Old 10-08-2017, 07:24 PM
DieselDonkey DieselDonkey is offline
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80mph here around 24 mpg I would say with my newer thermostat
Cut speed to 65mph my mpg goes up big time
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Old 10-08-2017, 07:44 PM
edycol edycol is offline
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Quote:
Originally Posted by DieselDonkey View Post
80mph here around 24 mpg I would say with my newer thermostat
Cut speed to 65mph my mpg goes up big time

I once tried 65mph for 80 miles, very slow acceleration, using momentum in slowing down, managed to hit 33mpg.
I never did that again since if it was my primary goal to save fuel I would be driving Prius not 425lb-ft tractor.


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Old 10-09-2017, 05:56 AM
smassey321 smassey321 is offline
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Quote:
Originally Posted by FredoinSF View Post
22MPG at 80 on flat terrain seems low indeed. Do you have Carly or some other diagnosis tool? You can typically monitor several parameters concurrently. Would be nice to see a speed RPM MPG line graph for the sake of objectivity.

FWIW I get roughly 24MPG for each tank. 80% of my driving is highway at 78MPH (actual, not BMW buffered value), but I am not on flat terrain so not sure how that works out as a comparison to your situation since logically mileage drops going up but then improves coming down the same hill on the reverse trip.
The remaining 20% is in town driving and I have steep hills there too.


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I have all the test tools. ISTA+ Carly, Testo. Let me see what I put together. Regarding my sea level comment, I forgot about the forced induction. Also, most time I don't reset the MPG at fill ups because it does not change anything. Thanks for all the info.
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  #16  
Old 10-09-2017, 06:09 AM
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Doug Huffman Doug Huffman is offline
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Quote:
Originally Posted by smassey321 View Post
Also, most time I don't reset the MPG at fill ups because it does not change anything. Thanks for all the info.
Another indication of a potential problem. A reset should change something!
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Scepticism and Animal Faith (1923)
Scepticism is the chastity of the intellect, and it is shameful to surrender it too soon or to the first comer: there is nobility in preserving it coolly and proudly through long youth, until at last, in the ripeness of instinct and discretion, it can be safely exchanged for fidelity and happiness.
(The Works of George Santayana p. 65)

Eschew eristical argumentation. I am responsible for what I write, not for your understanding of it.
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  #17  
Old 10-09-2017, 06:18 AM
smassey321 smassey321 is offline
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Quote:
Originally Posted by Doug Huffman View Post
Another indication of a potential problem. A reset should change something!
Just to be clear, resetting actually resets the setting. It ultimately ends up in the 21-22 MPG range after a hour or so. I am 100% sure something is not right elsewhere.

Also, I found my CBU pictures from 18 months ago. https://xoutpost.com/bmw-sav-forums/...hould-wet.html
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Old 10-09-2017, 07:30 AM
TroubledGnome TroubledGnome is offline
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Originally Posted by edycol View Post
I once tried 65mph for 80 miles, very slow acceleration, using momentum in slowing down, managed to hit 33mpg.
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I was able to do similar, but it was because I was forced to follow someone that would only do the speed limit.

26mpg is average for me in light traffic and cruise set at 80.
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  #19  
Old 10-09-2017, 07:40 AM
nosignolife nosignolife is offline
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27-29 is around what I get from my 2007, swirl flaps removed and 108k miles


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Old 10-09-2017, 08:19 AM
smyles smyles is offline
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27-29 is around what I get from my 2007, swirl flaps removed and 108k miles
different gallons, mate
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  #21  
Old 10-09-2017, 10:57 AM
edycol edycol is offline
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Originally Posted by TroubledGnome View Post
I was able to do similar, but it was because I was forced to follow someone that would only do the speed limit.



26mpg is average for me in light traffic and cruise set at 80.


Here in the West speed limit is bit higher, plus no one wants to pay taxes, so no police like in the East.


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  #22  
Old 10-09-2017, 11:49 AM
TroubledGnome TroubledGnome is offline
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Here in the West speed limit is bit higher, plus no one wants to pay taxes, so no police like in the East.


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Many of the east coast highways are now posted 70 mph. The rest are mainly 65 mph. With the error factor in the speedometer no one is going to get a ticket driving at 10 mph over the posted limit. Other than that, there is always someone out on the road that needs to show you that his car is faster than yours. So cruising at 80 mph is never much of an issue other than dealing with Virginia and Jersey drivers. ; )
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  #23  
Old 10-09-2017, 04:52 PM
RPsX5d RPsX5d is offline
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MPG Held Steady at 22.5 mpg over 100K miles then a step drop to 18.5 mpg . . .

For the longest time - 100,000+ miles over six years - my mpg hovered around 22.5 mpg (60/40 city/hwy). Yes, on the highway cruising around 80-85 mph it was still the same . . . but if I dropped the speed to about 65 mph and engaged the cruise control, mpg climbed to about 27 mpg.

About a year ago, over a period of a month and a half (that is when I noticed, might have been shorter) mpg dropped to 18.5 mpg . . . and it is stuck there for the past one year plus (about 20,000 miles).

I have the staggered 20-inch wheel configuration with RFTs. I also have the 3rd-row seating.

35d, MY2011, June 2010 build.

Spent a ton of time trying to figure out what happened, still stumped. Eliminated all the obvious ones like summer/winter diesel, tried different brand diesel (for the first 100,000 miles it was Chevron diesel from pretty much the same gas station), under-inflated tires, thermostat holding steady 87°C, etc etc . . . all checked out.

My current *guess* . . . one or more of these components are on its way out - MAF sensor, low-pressure EGR valve (electro-pneumatic valve), high-pressure EGR valve . . . something is causing the engine to run rich (i.e consume more fuel) . . . I wish there was a way to test for it.

The other possibility is - WAG - increased load as in increased rolling resistance . . . if this was the case, by now (20,000+ miles) it would have kicked out a fault code.

With no fault code, tough to figure this one out. Swept for fault codes using Carly . . . none.
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  #24  
Old 10-09-2017, 05:11 PM
DieselDonkey DieselDonkey is offline
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What are your coolant temps?
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  #25  
Old 10-09-2017, 06:46 PM
Kostyan Kostyan is offline
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Assume something may play a role in this - I am not getting some stupid 30mpg like people may claim and 27-28mpg literally on pure highway without a single street light, and at 70-75mph.

Given there are no issues with the powertrain (clean filters etc) have you checked to make sure that none of the 4 calipers are frozen or there are no problems with wheel hub/bearings? Try to touch each wheel one by one after prolonged driving; one that's too hot to the touch may have a problem.
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