BMW X3 Engine Problems
How to reduce fuel consumption in an X3 20i
Your fuel consumption is way off the claimed consumption figures released by BMW. The company claims the X3 20i should get on average 7.3L/100km, 6.2L/100km on the highway, and 9.2L/100km in city driving. You have to allow for some variation on those numbers due to individual driving conditions, but to be so far out suggests there is something wrong with your car. I wouldn't expect it to reduce enough to get even close to those numbers simply by driving it. Have it checked by your BMW dealer.
BMW computer
Yes, I agree, it is unreasonable. Take it to a BMW specialist, like John Heckrath at Heckrath Engineering in Melbourne and he will be able to reset the computer for you. BMW change the oil on the basis of engine hours run, not kilometers driven, and the computer needs to be reset to start the clock ticking towards the next change.
