[wistarf]

It's definitely doesn't reset these when I zero the trip computer.

Also, it also doesn't simply post the average over the last 18 miles like it does with the miles-to-go display - or my mpg would jump dramatically during the first 18 miles of a highway trip (I drive 95% city).

It seems to be averaging the mpg over some interval, but I've not been able to figure out what that interval is.

[ironic]

i'm not sure either, but i know u can reset it

[Kamando]

Page 68 (idrive version 335ci owners manual):

Average speed
Periods with the vehicle parked and the engine
switched off are not included in the calculations
of average speed.
With the trip computer, refer to page 69, you
can have the average speed for another trip displayed.
To reset average speed: press the button in the
turn indicator stalk for approx. 2 seconds.

Average fuel consumption
The average fuel consumption is calculated for
the time during which the engine is running.
With the trip computer, refer to page 69, you
can have the average consumption for another
trip displayed.
To reset average fuel consumption: press the
button in the turn indicator stalk for approx.
2 seconds.

[SoYank]

It appears to me to be the average fuel consumption over the miles since the computer was reset. If I reset the computer at the filling station and start off it will start with something like 6 mpg and increases as I'm moving until it reaches around 24-26. If I reset it while moving I get a ridiculously high reading that over a couple of miles will decrease to a more reasonable amount. Here's a picture to illustrate this:

[poiney]

SoYank, I'm confident that you are correct. One can easily tell that the dash display continues to approach a steady-state value for longer periods since the last reset. I've noticed very different driving habit over 18 mi, and the "mpg avg" remains relatively flat.

BTW, I haven't rest my trip computer that displays in the assistance window. Now that I have averaged 2k miles on that one, it displays 24.0mpg permanently.

[mike fab]

Hmmmm. Mine seems to always read more like 11mpg. Maybe my gauge doesn't work? Hey, at least I'm having fun...

[BK]

Originally Posted by wistarf

It's definitely doesn't reset these when I zero the trip computer.

Also, it also doesn't simply post the average over the last 18 miles like it does with the miles-to-go display - or my mpg would jump dramatically during the first 18 miles of a highway trip (I drive 95% city).

It seems to be averaging the mpg over some interval, but I've not been able to figure out what that interval is.

I was going to say I remember reading "18 miles" but then your post above clarifies where I must have read that -- from the miles-to-go display. It would make the most sense to have this from the last reset. I use it to track my mpg and mph for each tank of gas. Maybe it will wear off eventually, but for now, I'm that obsessive.

[poiney]

I don't know what is preferred for the miles to go display.

For me, the primary advantage is that it is easier to think in miles to fill up, rather than percent tank to fill up.

Insomuch as the 18 mi is totally arbitrary, it seems more logical as you suggested to the let the driver decide when to reset to give the best estimate to next fill up (such as reset during a tank if one goes on a long trip since the mileage will be much better).

[LEDZEP]

Another easy way to reset them is to select BC in your audio menu. This will display the trip information. Select the button under Avg MPG and then select Reset... same for Avg Speed. It's a little faster than using the stalk, although either one will do.

Basically, the averages continue to accumulate between manual resets, so if you have driven 5,000 miles since a reset it will be averages for the past 5,000 miles, etc. I do mine every fill up.

[Shawn_speed]

I don't have iDrive. I am in the habit of pushing the button at the end of the turn signal lever as soon as I fill up for each category to be displayed (Avg MPG, Avg MPH). Once in the display, I hold the button until it resets. When the tank is empty, I compare the two figures (Avg MPH vs. MPG). This comparison is important because it tell me in what conditions the MPG was achieved. Congested city driving brings the avg. MPH way down, so I take that into account looking at my gas mileage. On the other hand, averaging over 95 miles an hour on the highway gets slightly less gas mileage than averaging 65 which is again relevant.

The miles to an empty tank is based on the last 18 miles average fuel economy. There is a good reason this is not user selectable. If it was user selectable and you set it for 50 miles and forgot it, the information about when the tank would be empty would be meaningless when you are down to 20 miles to go and you are searching for an open gas station. Lets say you have exited the freeway around downtown Los Angeles and you are looking for a gas station. The mileage you are getting would be totally different than the 50 miles average while you were on the freeway.

Shawn

[wistarf]

OK, thanks everyone. It makes sense that it keeps accumulating the average between manual resets. What I was missing was how to reset it - apparently I somehow missed that section in the manual.