I agree, amacento. There is only one correct way to pronounce it -- the way Karl Abarth himself, did. And it didn't matter whether he was in Italy or Austria. His name is pronounced "AH bart". The accent is on the first syllable.
My own name is not very common. Consequently, I get seriously POed if people can't say it correctly, once they've been told what's correct. There is only one correct way to say my name, and it's not difficult to pronounce. Once instructed, it's an insult if anyone's own name isn't pronounced properly. Poor Karl/Carlo is dead, but I still consider it insulting to pronounce his name incorrectly!
Jay Leno, and hundreds of others, including many of Fiat's early marketing personnel in the U.S. had/have no clue what's correct, so they try to say "Abarth" quietly and quickly, trying to not call attention to their stupidity/insecurity.
I have a 1% portion of respect for those who say "I paid for it -- I can call it shyte if I want". Yes you can, but you are insulting the person whose name is on our cars, even if he's actually dead. But I don't have an ounce of respect for that kind of attitude.