You sure about that? How are you computing the mileage? If you are trusting what the trip computer is telling you, its probably incorrect.
The calculation should not be done against "tank size"... it should be against the actual number of gallons that it took to fill the tank.
i do a bit of both and driving how the car should be driven, i get about 21-22 MPG, but i have gotten 32 doing all highway with the cruise set at 90 MPH on the speedometer, 81 actual speed
Something is wrong with your speedo or you're rolling on really small wheels to be off by 10%. BMWs do read optimistic but it should only be about 3-5% off.
i have 40/r17s on the front, 45/ 17s on the back so i dont think thats the issue.. but when im driving by one of those things that tell you your speed at 30 it will say 27, 40 it will say 36.. is there any way i can get it professionally checked without spending an arm and a leg?