We've owned our X3 since around 2/2004. Great car and now has some 97,000 miles. I've always had it maintained at local 90266 BMW independent who has always done great. This is my wife's car and her driving style and habits haven't changed, and her daily runs around town are the same as always. But some years ago, the mileage started to get really bad. Earlier in its life the X3 got a combined 17 - 18 mpg. Since she doesn't put that many miles on it, I sort of ignored it. Also some years ago, we kept getting a check engine light, and correlated that to my wife's increasing inability to tighten the original gas cap. So we replaced the cap and haven't gotten that fault since then. Now its our only car and I've not been able to have the cause of poor mileage diagnosed. Lots of really clever ideas, but none of them pan out. I might say that I know a LOT about cars in general, but I can't figure it out for sure. I've checked the mileage manually ala' miles driven versus gas used, and the computer is pretty much right on. I've been doing some testing. If I'm driving around at 30-40 mph and reset the mileage tracker, I get a great 25+ mpg. On the freeway it's great as well. But after I've done a driving speed reset, if I come to a stop light, you can visually see the mileage literally plummeting at idle. Using a ton of gas at idle!! I'm told that the fuel pump puts out constant volume. At idle, there is supposed to be some kind of "return" line that sends "unneeded" gas back to the tank. If that's true, then it doesn't appear to be working. And there's never any sign of or smell from gas leaking. So why is our old 2004 X3 "using" so much gas at idle?