What does si mean in quarter horse racing?

Question

when you see a quarter horses stats itll say something like si 105 or si 99.

Answer 1

SPEED INDEX....OR SPEED RATING.....MOSTLY DONE AT FAIRS, INSTEAD OF CLASS, THEY GO BY SPEED INDEX.....

MOSTLY QUARTER HORSE, OR MULE RACING

Answer 2

Speed Index.  It's a way of measuring how fast a horse ran while making some allowances for weather, track conditions and the like.

Answer 3

Speed Index