explanation here:
"no speed limit" means that there is >no< speed limit, i.e. go as fast as you want, police won't be able to fine you because you're allowed to drive at any speed legally
"unlimited speed limit", while not entirely grammatically correct, means that the speed limit is unlimited (i.e infinite, even if infinity isn't really a number), so the maximum speed allowed legally is infinite, something that no car can reach, so, once again, go as fast as you want, because you can never reach a speed that is illegal, thus cops can't fine you because you >are< within the limit
that's why they're basically the same thing, even if the details do differ a lot
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