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I think it depends on the track you go to. This is from the website of the strip I go to.
Xtreme-Street 6.00 to 6.41
Pro-Street 6.42 to 6.99
Modified Street 7.00 to 7.49
Super Street 7.50 to 8.99
Fast Street 9.00 to 9.99
Street Stock (no slicks) 10.00 and above
That is how they class cars. But it is for an 1/8th mile strip and is most likely just their own numbers. Other tracks probably have a similar system. Just contact the one you plan to go to or look at their site.