Yeah, I wonder about this too sometimes. I think it could be some or all of these factors: Factory wheels/tires(either stock or taller and skinnier), lack of f/r "IS" spoilers, synthetic fluids, great condition ignition system(coils,wires,plugs), great condition charging system(alt, batt, wiring), new air filter, good condition rear diff/axles/hubs, properly mounted rear muffler, good condition cat converter.
Some of these items can be very miniscule in effect but nontheless, still valid. While other items can be hard to diagnose because they don't necessarily break right off the bat but merely just wear down gradually, like coils or a cat converter.
I seemed to notice guys in non-snow states seem to be fairing better in mileage. This leads me to believe that possibly some grounding points can be partially corroded not giving maximum electrical contact, thereby possibly offsetting any values sent back to the ecu or not making the charging system as efficient.
Those stupid grounding kits you see sold for all the cars might actually be the most beneficial on our cars. I'm going to be designing my own and see if it makes any difference in mileage.
My car IS in great condition, rebuilt motor, new rear muffler everthing works excellent, new filters, plugs .....and all the other stuff you said. but i still cant get over 30mpg, im not boasting or anything here but i am an aeronatical engineering major and i know a thing or to about inertia, aerodynamics, heat transfer etc....i havent really done the math but just by think about what i know, it is not feasible to get 35+mpg in our cars with the factory wheels and diff. by the way im also running 15" wheels with 205/55-15 tires which "should" help freeway gas milage, im running a tire that is also 1 pound lighter than the stock tires assuming most of you are running 195/65-14's in a decent tire.
also, on your not to the snow state guys, cold air will make your motor run slightly better, its more dense. but it also creates more drag....unless, you live at a higher altitude, then you will have air that is very "aerodynamic". with all this said you will actually see better gas milage at sea level, even though the air is more dense and harder to push a object through. i can explain it better if you guys dont understand, im not to good with words, when explaining this stuff.
one more thing: those magnets and thicker ground cables will not do anything to improve gas milage, if magnets are so good for the car they would have done this at the factory, and if the cable on our car is not "big enough" (as the ebay scammers say) then BMW would have put a bigger cable on there at the factory also. in all these years BMW's have gotten more and more gadgets and, believe me, the ground cables are still the same size. Save yourself some money and dont buy that ebay crap, a big ground cable will only help if you have a larger stereo.
