Last time I checked, I was getting around 18 mpg overall. The computer, which I don't use anymore was showing 24 mpg.

 

I think many of us reach conclusions based on information that is not comprehensive enough. I started off keeping a record of every fueling operation in my 545. I immediately topped up after driving less than a quarter mile from the dealer when I picked it up. It was already showing full, and I zeored the OBC. I have continued to zero the OBC for each tank and compare to the mileage based on each fill up. My last refueling was over a month ago at 6186 miles. Here is what the accumulated record shows.

OBC = 22.32 mpg
ACT = 21.42 mpg
Diff = - 5.3%

Even this information cannot be conclusive, because a couple of months ago it looked like the difference was settling down on -4.0%

Incidentally, out of curiosity, I have been doing the same thing with the X3, although I missed keeping the record on the first tank. Subsequent tanks for about half the mileage that is on the 545 show the following.

OBC = 24.21 mpg
ACT = 23.53 mpg
Diff = -2.9%

Both vehicles weigh approx. 3800 lbs. One has the 3.0 liter engine and the other 4.4. There are other differences, too, that would affect mileage, not the least of which is the driving pattern on the X3 has a high percentage of highway miles at 80+ mph cruising. With comparable driving, I believe the 545 would equal the fuel economy of the X3.

 

Around town, meaning stop and go surface streets and crowded expressways, I get in the mid-teens, all that with fairly aggressive driving and my stick.

On the highway, if I reset the mpg, I get maybe 30 or 31 at a very mellow 65 mph in 6th gear, and that goes down to about 26 or 27 going 78 mph. So it gets nice mileage on smooth, moderate highway runs.

I think the mileage needle thing, as you approach a stop, just measures worse and worse mileage, which is true (I you really went that slowly, the mileage would be crummy), and then when you're not moving it feels it can no longer measure it. Now that you mention it, it should probably peg on the zero end, not the infinity end, when it's stopped.

 

The needle is set more like a manifold pressure gauge. The more pressure (deeper the gas pedal position) the more manifold pressure and the poorer the fuel economy. There would be minimum manifold pressure at idle with the maximum fuel economy when the engine is turning over - not the wheels. I have no idea whether the gauge is actually measuring manifold pressure or not, but it sure works like it is.

And one for Rudy. I know you hate the gauge, but I miss the fact that the X3 does not have one. But, then I drive for mileage a lot of the time and I'm betting that you don't.

 

In my 545i with Sport Package, I am getting about 18 mpg in city like stop and go traffic (round trip to/from work is about 16 miles with many stop lights and stop and go on the "freeway" in SoCal). I have never gotten more than about 23 mpg on pure highway driving, but I have not driven more than 15-20 miles on the highway at a stretch before I'm back in city traffic with stop lights, etc. I expect the highway mileage to go up significantly on a long road trip (say more than 100 miles before exit), perhaps to the 26-28mpg range.

As for the mpg needle, I observe the same behavior in that you get minimum mpg while the Steptronic downshifts prior to a stop, when the needle then swings to the maximum mpg. I do not believe the needle is measuring some kind of manifold because you can cause it swing to maximum by braking hard instead of allowing the Steptronic to downshift and slow down the car (you should try this). I believe that the needle somewhat accurately reflects fuel consumption when slowing down because of the close gear ratios on the Steptronic cause significant engine braking when slowing down on a 3800 lb vehicle -- if you brake hard (but not to a stop), then the car will coast a little when slowing down before Steptronic engages again and the needle swings to minimum mpg.

If you manually shift the gears (instead of D or DS), you can leave it in a higher numbered gear to nullify the engine braking and you will see the needle swing to higher mpg. It's the trade off of having more aggressive gearing for improved acceleration with the Steptronic that causes significant engine braking when slowing down.

 

I have about 1300 miles, and my results / observations match yours exaclty, including the swinging needle.

When I make a real effort. by keeping therevs down and the pedal up, I can get my around town milegae up to about 15 and highway to about 22. But, driving as I usually do, my numbers match yours.

 

After 5 Tanks of Fuel and 1541 on my car, I am averaging with a low tank of 14.07 and a high tank of 19.37mpg. The car routinely gets around 17.5mpg. I mainly do city driving with about 25% freeway mixed in.

I have noticed that each tank has improved in MPG.

My MPG Meter swings like yours at stop signs. All I know is most of the time, mine is too the right, as I am stomping it!

Regards, Mike

 

Now we know the real reason for the low gas mileage!

 

What you have described is exactly the way an intake manifold pressure gauge would work. I am not saying that the BMW gauge is necessarily a manifold pressure gauge, just that it mimics one.

 

DR.Z: You don't say what year your car is or if you know the software version. I had a similar problem where the OBC was showing 23 mpg and I was actually getting about 18. BMW gave me some song and dance about the OBC using a complex algorithm to calculate the mileage. All I can say is that once the software was upgraded to version 12, the OBC mpg became much more accurate, only off by 1 mpg or so.

For what it's worth, my 530's mpg has improved considerably over 14,000 miles, the last 10 months. Hope yours does too, as I am taking ED of my new 545 on Jan. 3 2005.