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Cost: $18 (eBay version, TRD versions are more than $100)
Time: 1 hour
Theory: A short shifter has a shorter "neck", thus making the movements to shift shorter in range. This allows a driver to save fractions of a second on every shift.
Opinion: A lot of people buy a short shifter as bragging rights. I will openly admit I bought mine more for the looks: I had a TRD knob and boot. The boot did not come up to the knob, leaving some on the metal shifter showing. This might save a fraction of a second here and there, but more importantly, I think, is that the feel of shifting is much more firm. The shorter shift movement just "feels better".