SI

Followed by a number is the Speed Index (the speed rating) of a horse. The SI number often is listed after a horse’s name, like My Best Friend SI 90. A horse receives a speed index number every time he races at an AQHA-recognized track. The speed index system was developed as a way to compare horses in races run on different tracks. Ratings in the 70s are considered good, 80s very good, 90s fast and 100+ very fast.