how to pronounce numbers

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This thread reminds me of a Code Golf contest I participated in a few years ago. (The results are still online, by the way.) The challenge included writing numbers using letters, in English language. In the private messages between the guy who posted the challenge and me there was some disagreement over the letters used to represent 40. He said "forty", my program produced "fourty" (which I perceived as correct) as a side effect of my optimizations (in code golf the size of a program determines its score; this particular optimization used the same list of strings for -teen and -ty). As I'm typing this post, my spellchecker is marking my version as wrong (and it marks itself too - oh the irony), but as I'm not the only contestant who put the u in there (e.g. the Java entry that isn't mine explicitly contains the string "fourty" which could have been altered without side-effects other than an improved score), I guess this isn't as easy as the spellchecker makes it appear. In school I was taught "fourty" too, but this is not an English-speaking country, so that alone doesn't mean a lot. Regional differences again or what ...? Could a native speaker please enlighten me?
(Okay, that was about spelling instead of pronunciation, but close enough.)