The answer is directly dependant on the tire that is being discussed. Maxxis tires measure smaller than Nokian which measure different than Michelin etc., etc.
To somewhat answer your question, I've run Nokian Gazzolodi Jr. 2.6 and Maxxis Minion 2.7 on my 2005 Firefly plus with no problems however, there was a noticeable difference between the clearance of the two tires with the Nokian having less room.
What size tire will clear on a firefly/flick? Is the clearance different for different years?
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