Shift knobs are identical through the years. The only different is a switch made into it and plug hanging out the bottom.
MPG drop will be minimal, pending how heavy your foot is. If anything it should stay the same due to the clearer exhaust.
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'13 Mustang 3.7V6 - MT82
'00 GP SE (L67 Swap, XP, Headers, 3.25")
'98 GTP, '01 Bonne