I don't think it's that unusual. There are a lot of variables to consider. First and foremost, the way gas mileage is determined is an antiquated system, so you will hardly get the rated mpg. It can also depend on what type of fuel you're buying. Many folks refuse to buy premium gas, but they don't understand that lower grades can actually lower your miles per dollar . . what you put in is what you'll get out of it. You may have innaccurate or malfunctioning sensors/components that are throwing off the air/fuel ratio. If you suspect this, throwing money for an inspection could save you $$ in the long run, and avoid bigger problems later. And lastly, BMW isn't exactly known for their fuel economy. And a half or full tank isn't going to matter; Kistoni is right- the fuel keeps the pump running cool, you should never wait too long to gas up. Hope this helps!