The original system used a letter grade, starting at D, then C, B, A and the highest AA.
[1] 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.
If the average doesn't meet the minimum standard time given by the AQHA for that distance, then the minimum standard time is used instead of the specific track average.
To calculate a specific speed index for a specific horse in a specific race, the time the horse finished in is compared to the averaged speed for the distance the race was, and points are added or subtracted based on whether the time was faster or slower than the average.
The number of points subtracted or added varies according to the distance of the race, and is based on a chart put out by the AQHA.