I don't think I've seen it concisely written elsewhere, so I though I'd make a post of it.

Please sticky it if the info isn't there already.

The stock size for the 2nd Gen Fit Sport is 185/55-16

If you want to go wider, but still stay with 16":

205/50-16 is perfect

215/50-16 is +5.7%

215/45-16 is -5%

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If you want to go wider but go Plus One to 17"

215/40-17 is perfect

205/45-17 is +6.5%

205/40-17 is -5.3%