Famine
The extra S changes the stress from the I to the S. It goes from Ni-San to Nis-san.
In English, yes, but not in american English or Japanese.
In Japanese, the only consonant that can be used without a vowel imediately following it is
N. The first S in "Nissan" breaks this rule, obviously, so it's inapplicable to the Japanese pronunciation of the name, and is more than likely a slightly goofy result of romanization (kinda like the spelling "Mazda" -- the company's name is actually "Matsuda," but that was deemed "too hard" for westerners to understand, so the "Z" was substituted).
What you're left with is "Nisan," and since the Japanese "i" is pronounced like a long "e," and the Japanese "a" is pronounced like the "a" in "father," the Japanese pronunciation is "nee-sahn."
Similarly, the Japanese pronunciation for "NISMO" should follow the "long e" rule for the "i." What about the S without the vowel, you ask? Well, in this case the Japanese pronounce it with an ever-so-slight "u," which happens to be the weakest vowel in Japanese, and is often made (practically) silent. Thus, NISMO is pronounced, "nee-s(u)-moh," which ends up sounding like "nees-moh."
The american English pronunciation mirrors the Japanese one (I know, it's hard to believe, but it's true

) -- "Nissan" is pronounced "nee-sahn."
On the other hand, americans pronounce NISMO "niz-moh," and I often pronounce it that way myself out of habit.
