Speed index

Speed index (sometimes Speed rating) is a system of rating the performance of Quarter Horse racehorses. The American Quarter Horse Association (or the AQHA) has used two systems over the history of Quarter Horse racing to evaluate racing performances. The original system used a letter grade, starting at D, then C, B, A and the highest AA. Later AAA was tacked on the top, and later still AAAT (for TOP AAA) was made the top speed. Eventually, this system became too cumbersome, and a new system was introduced: the Speed Index system, which used a number system, with 100 being roughly equivalent to the old AAAT. This change occurred in 1969.
The actual calculation of the speed index starts with taking the three fastest winning times at a particular distance for the past three years at a given track. These times are averaged together, which is then the 100 speed index for that distance at that track. If the average doesn't meet the minimum standard time given by the AQHA...