Question: Why did Zeus and John steal a Yugo to chase the dump trucks? Zeus abandoned the taxi outside the Federal Reserve (in fact you actually see the Yugo about 100 yards back from the taxi when the bomb explodes) wouldn't it have made sense to use that again considering it'd be faster?
Answer: If you look as they drive off in the Yugo, the cab had actually gone. It could've been moved as part of the cleanup operation or given that Zeus probably left it running in his hurry to get to the phone, stolen by someone.
Answer: The bomb was going to get off anyway, it didn't depend on them not making it in time. I don't think Simon cared much if they had made it or not. If they did, the bomb would have blown up anyway, and the game he was playing with him would simply be over. He'd probably have McClane killed right after that, before he might figure out the hows and whys.
lionhead