my gas mileage has dropped. What's causing it?

The Ethenol and fuel in general is the main issue.

Don't hang parts on it until the problem goes away - you will spend a lot more money than if you just had it diagnosed properly. Fuel filters are rarely a problem nowadays - you're thinking 25 years in the past. Actually, tire pressures are he number one cause for degradation in fuel economy, but in your case there is a good chance that your "engine overhaul" was done poorly - why was it overhauled - overheat or extremely poor maintenance - had you only done some competent maintenance... You should take this very sophisticated, technologically advanced piece of engineering beauty to a trained BMW Specialist (a specialist is someone who only works on one make of vehicle). There are many qualified independent, as well as dealer choices out there. Remember, there is no such thing as a Euro Specialist, or a Japanese Specialist, or a Domestic Specialist, or even a German Auto Specialist - the term would be an oxymoron. A German Auto Specialist would have to be proficient in seven different makes - this is not possible anymore "Jack of all trades, master of none". You can look up an excellent BMW shop by going to the BIMRS web directory (non-profit) at bimrs.org