So earlier, we lovingly took our Tinseltown-trashin' brother Defamer to task for calling Tobey Maguire's 7-series a "Beemer." We also admitted that we had no reason why BMW's bikes are called "Beemers" while its cars are known as Bimmers. The etymology of the bike thing is pretty easy post WWII, BMW was primarily known as a motorcycle manufacturer, and one of the marque's main opponents on the track was BSA, whose bikes were colloquially known as "Beesers" (or, as we see it in American cycle rags, "Beazers"). "Beemer" was simply a similar nickname attached to the Bavarian bikes to achieve some sort of friendly parity with the machines from Sheffield.
The deviation in spelling for the cars, on the other hand, is a bit murkier. As best we can figure, the car guys didn't wanna be associated with the motorcyclin' hoodlums. So despite the fact that the names sound identical, apparently a vowel swap was enough for the four-wheeled motorists. We still don't get it, but that's the way it is. [Thanks to the Dubba-dubba-dubba-dubba-dubbaya Bee for the tip.]
Bimmer, Beemer or Beamer? What is in a name... [The Ultimate BMW Blog]
More BMW [Internal]