I have what they call a habitual speeding problem :devillook
I usually average a speed of 80 MPH and get an average 21.3 MPG. Got to love living in open places outside the city. From Carson City to my house burns a hash mark of fuel on the indicator.
I was following the boss home the other day for a meeting, so I was on my best behavior doing 50 MPH, and burned through 3 hash marks while only adding 2 miles to the total trip.
So is the car more efficient at higher speeds with the lower drag coefficient? Is BMW saying that it's okay to speed by making a car that is happy at 80MPH as opposed to 50? :redspot

Could be the road had more hills or something, the best mileage is around speed limit as many of my long trips have revealed, not that this makes me drive speed limit!

'08 328xi - I drive about 10 miles round trip to and from work every day, and it's probably 75% highway (ranging from stop and go traffic to full 70mph traffic), and then 25% city driving.

I've put on 700 or so miles so far on the car, and my average speed is around 25 mph, with an average miles/gallon at around 18.5.

I was hoping for at least 20mpg

I have what they call a habitual speeding problem :devillook
I usually average a speed of 80 MPH and get an average 21.3 MPG. Got to love living in open places outside the city. From Carson City to my house burns a hash mark of fuel on the indicator.
I was following the boss home the other day for a meeting, so I was on my best behavior doing 50 MPH, and burned through 3 hash marks while only adding 2 miles to the total trip.
So is the car more efficient at higher speeds with the lower drag coefficient? Is BMW saying that it's okay to speed by making a car that is happy at 80MPH as opposed to 50? :redspot

I assume the 21.3 is just highway at 80 MPG not all your driving? You should be getting much better than that. All highway at 80 MPH I can get 26-28 MPG depending on conditions. Even though you have an X it should not be that much worse.

'08 328xi - I drive about 10 miles round trip to and from work every day, and it's probably 75% highway (ranging from stop and go traffic to full 70mph traffic), and then 25% city driving.

I've put on 700 or so miles so far on the car, and my average speed is around 25 mph, with an average miles/gallon at around 18.5.

I was hoping for at least 20mpg

I usually get around 20 mpg with each tank. My driving is suburban/city with short highway trips 3 miles, 5 miles, etc mixed in. I am at 16,000 miles so maybe you will see some improvement when fully broken in.

Do you have the step of 6MT? If step, do you have it in DS or auto? If MT, are you in 6th most of the time or do you tend to stay in low gear. In all fairness unless you are driving the exact road and similar conditions (no winds, etc), you are not comparing apples to apples. Best way is to take the same stretch of highway in the highest gear and try different runs at different constant speeds then compare. As I said, to me the lower speed yielded better economy in such circumstances.

I got over 30 mpg based on the 335i sedan car computer average on I85 for 800 miles with cruise control on 75 mph. Have to admit this is too good to be true. Next long trip I will calculate against the car computer.

Even the Prius will yield lower MPG at 80mph vs 50mph...and its drag coef kicks the 3's ass. I think there are several factors at play, some of which others have mentioned already:

1) Driving style - If, when you're at 80, you cruise smoothly and the time you went 50, you were constantly gassing and braking, you'll see your economy go down.

2) Gear - If you drive a stick and you're not upshifting soon enough, economy goes down. Btw, I usually end up in 6th when I pass 40mph. I like to accelerate fairly hard and I still end up getting about 23.5 MPG in combined city/hw.

3) Air conditioning/windows open - Small but tangible negative effect on economy.

4) Anything you've added to the roof - Your economy plummets if you add any roof accessories.

5) Tank fluctuations - My capacity reading can changed a lot depending on the slope of the hill I'm on. And it doesn't immediately change when I get level again.

6) Probably a bunch of other stuff I missed.

Basically, your sample size of 1 is way too small to extrapolate anything meaningful. I can tell you this though...if ALL OTHER variables are equal, you will get better economy at 50mph than 80mph. You can't defy physics.

I have what they call a habitual speeding problem :devillook
I usually average a speed of 80 MPH and get an average 21.3 MPG. Got to love living in open places outside the city. From Carson City to my house burns a hash mark of fuel on the indicator.
I was following the boss home the other day for a meeting, so I was on my best behavior doing 50 MPH, and burned through 3 hash marks while only adding 2 miles to the total trip.
So is the car more efficient at higher speeds with the lower drag coefficient? Is BMW saying that it's okay to speed by making a car that is happy at 80MPH as opposed to 50? :redspot

I have just filled up the tank for the second time (~800 miles on car now). I had 21.9 MPG with mixed driving.

It was 1 hash mark on the gas that I burned going 80 MPH. I top off in Carson every time because gas is .30 cheaper a gallon than purchasing in Lake Tahoe with mark up an CA taxes. This trip going 50 was 4 hash marks. Close enough to a quarter of a tank to be frightening. If you are familiar with this stretch of road, it is HWY 50, Spooner Summit. All uphill, windy and what driving is all about. I have a manual transmission, and keep it in fourth gear on the uphill. When I speed, I'm between fourth and fifth. I like to keep the RPM's between 3000 to 4000. Doing this at a slowers speed burned more gas. The 3 is like an inverse Prius, the faster the more efficient. The Porsche is like that too. It's sweet spot for consumtion is 3500 RPM's, before the power band kicks in, 5th gear, 80MPH. It gets a whopping 18 MPG, for peak efficiency, before the first oil crunch in the 70's, ha ha.
My 21.3 MPG number is the aggregate average I get on my commute from Tahoe to Reno, 54 miles. Through it all I get a windy road segment, 5 miles of stoplights, and open mile highway driving, and a 50 zone from Hell entering Reno proper (I'm glad I have a Passport 9500.) In all, a great cross section to get a decent real world MPG average. when I top off, I cruch the numbers for a rough MPG and compare to the car computer, pretty close.
These cars are made for the autobahn. I'm convinced the are made to be more efficient at higher speeds than we are allowed to do in the US

Pfft, you are comparing an uphill drive to normal commute, that monkey should slap you silly :stickoutt.

I'm not convinced that all engines are more efficient at lower speeds than higher speeds. Of course, if someone here wants to post references that prove it somehow, I'm always willing to read. :)

I believe (but can't prove) that engine efficiency involves many factors. Some of the ones I can see that play into the MPG question would be: the RPM range where the torque is the highest but fuel consumption is the lowest, engine operating temperature, coolant system efficiency, oil circulation system efficiency, valve efficiency, and so on.

I have experience that says the 335i engine has a "sweet spot" at about 75-80 mph (for the AT in auto), since I've done the run from Tampa to Jacksonville multiple times and that speed range has given me the best mpg (31.1 on the computer). On the flip side, that was the best I've ever done on that route; more typical is 28.5-29.0. I chalk up the difference to headwind vs. tailwind and air temperature.

I'm actually comparing only the uphill drive. Typically done at 80 MPH, and I burn less gas than the single time I did it at 50MPH.
The rest of the description is solely for the average MPG on the car on any given day.
I still think BMW, Porsche, and probably any other german car is made to be more efficient at higher speeds than US.

So we can chalk most of it up to your driving style.

Because you like to keep your engine RPM in "the sweet spot," when you went 50mph, you ran the same RPMs except you did it for 60% longer due to the slower speed. Also, when you're traveling slower, you lose a lot more energy due to friction at higher RPMs because you don't have as much momentum (which is why enging braking is much more evident during slow speed/high rpm).

If you upshifted more or did that run with an AT, I'd bet that your fuel economy would've been much closer...possibly higher.

As for our cars made more for high-speed, I think there is some of that. But on the other hand, if you applied your driving style to a honda civic, you would see similar trends, if not as significant.

It was 1 hash mark on the gas that I burned going 80 MPH. I top off in Carson every time because gas is .30 cheaper a gallon than purchasing in Lake Tahoe with mark up an CA taxes. This trip going 50 was 4 hash marks. Close enough to a quarter of a tank to be frightening. If you are familiar with this stretch of road, it is HWY 50, Spooner Summit. All uphill, windy and what driving is all about. I have a manual transmission, and keep it in fourth gear on the uphill. When I speed, I'm between fourth and fifth. I like to keep the RPM's between 3000 to 4000. Doing this at a slowers speed burned more gas. The 3 is like an inverse Prius, the faster the more efficient. The Porsche is like that too. It's sweet spot for consumtion is 3500 RPM's, before the power band kicks in, 5th gear, 80MPH. It gets a whopping 18 MPG, for peak efficiency, before the first oil crunch in the 70's, ha ha.
My 21.3 MPG number is the aggregate average I get on my commute from Tahoe to Reno, 54 miles. Through it all I get a windy road segment, 5 miles of stoplights, and open mile highway driving, and a 50 zone from Hell entering Reno proper (I'm glad I have a Passport 9500.) In all, a great cross section to get a decent real world MPG average. when I top off, I cruch the numbers for a rough MPG and compare to the car computer, pretty close.
These cars are made for the autobahn. I'm convinced the are made to be more efficient at higher speeds than we are allowed to do in the US

Most BMWs (especially high performance models) are designed with the autobahn driving conditions as a baseline. I have tried several times to determine the highway mpg at various speeds, running full tank on the same road. The best mpg will be around 80-85.