In addition to Dan's info, there is a German/English pronunciation difference that contributes to the error. The word "Bimmer" is pronounced in English as a short I, as in BIG. In German, the letter I is pronounce as a long E. So in German, "Bimmer" (the car) is pronounced "Beemer". We speak english, so short I is the way to pronounce it.
And (in my useless opinion) since it is slang, the final "authority" is the BMWCCA, the unofficial authority on all things BMW. BMWCCA says Bimmer=car, Beemer=motorcycle.