Mercedes isn't a German word. It's Spanish and it's a name -- Mercédès, pronounced in Spanish something like Mer-SED-Ess. But we don't use the Spanish pronunciation in English. Or pronounce the names of German products with a German accent. We Anglicize foreign words and names, and if you don't believe me, I encourage you to visit Versailles, Illinois and ask the residents how they pronounce the name of their city.
Comments
Why is it different to German pronunciation? Logically to make it simple one would have kept the same pronunciation as in German.
In the process of Anglicization you have made the pronunciation unphonetic,
the point is why do not you keep the pronunciation the same and make it easier to pronunce as it is phonetic.
English is not a faithfully phonetic language. (pneumonia, knee, through, break/beak, debt, ...)
Where did you get that è? I've never seen it in Spanish. It looks more French to me that way, though, strange to say, the French use no diacritics at all on that brand name.
By the way, the German was [meɹˈtsedɪs], and the American version is [mɚˈseɪdiz]. The British version probably has a slightly different vowel in the first syllable due to the "r".
CJ