When I'm idling (800-900rpm), the analog meter displays no gas usage. But the digital meter in the dash always climbs up to the insanely low mileage. In my case 50L/100km, which is 4,7 MPG (US gallon). This is either insane or incorrect. Why would an idling engine need so much fuel?

How to test:
1) reset your mileage meter
2) stay idle
3) watch the digital meter as the number climbs

Because of this issue, average mileage is totally off. I stop at 3 stoplights during a normal trip and obviously the whole average is completely misleading from that point.

This does not happen to me...

It's correct. Think about it. You are not moving. Essentially driving 0 miles but burning gas to keep the engine running. You really should be getting 0mpg but because the car does an average over a period it'll show some number in your case 4 mpg

Simple explanation. I understand. However this causes the averages to be completely "inaccurate" (misleading is a better word) especially for European l/100km unit. It would be smarter from the BC meter to not count in any averages or switch to a hourly unit counting during idling. Maybe it's not the BMW to blame, but the algebra. Averages are "misleading". :rolleyes:

Try putting in chevron fuel injection cleaner in two full tanks of gas (a bottle for each tank) and see if that change your problem

My car only goes to either 39.9L/100km or 40L/100km. The problem is with reality and not the car. As stated, if you are not moving your mileage should be infinitely higher than even 40L/100. The only way to "more accurately" measure mileage is with a lot of miles when the car is moving; for example, after each fill-up and the mileage is a mix of idling and moving. The meter isn't bang on perfect but it does give a good representation of your mileage. After your next fill-up, reset the odometer and then when empty again use that fill-up to manually determine your mileage without the benefit of the computer. The figures aren't too far off but they will never be 100% accurate. You can't eliminate the stop portions because the fuel is still being burned. Thats why a hybrid gets good mileage since the engine is actually off for those times the car is stopped. I'm not convinced the 'moving' mileage in a hybrid is any better than other cars.