I have saw Caprices run mid 14's with bolt ons, and some Impalla's dip into the 13's with major bolt on's (gears & converter) so they are not that slow! stock however would probably be in the low to mid 15's!
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98'Trans Am LS1 273 gears Whisper lid,K&N,Whisper MAF ends,Flowmaster,Best ET on street tires 13.17 @ 106.49mph