Your speedometer will be a little off since your sidewalls will be 5mm shorter on each side. (10mm total for .4 inches less than original diameter) I forget which way this will affect the readout, but this is discussed often so a search should turn up lots of info.
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'98 SVT
KKM / TH fix / Mobil 1 / Synth tranny cocktail / JVC LX-30 HU / 10" JL sub / Alpine V12 amp / Panther 199 wheels / Kuhmo Ecsta 215/45/17