If want to know the actual speed of a horse, you can do it quickly if you know the span of time that a horse has run over a particular distance. Horse races are measured in furlongs; a furlong is an eighth of a mile. By breaking down the speed of the horse per furlong and extrapolating the speed over a mile, you can calculate the average speed of the horse over the course of the distance.
Convert the time for the horse over a given distance into seconds, if it isn't already, by adding 60 for every minute to the seconds portion of the time. For example, if the horse's time is 1:12, add 60 to 12 to yield 72 seconds.
Divide the time in seconds by the number of furlongs to get the average time per furlong. For a six furlong race, a 72-second time would be 12 seconds per furlong.
Multiply the time per furlong by eight to get what the average time per mile would have been at that speed. For example, 12 times 8 equals a 96-second mile.
Divide 60 by the time-per-mile from Step 3 to get the percentage of a mile the horse would cover in a minute. A 96-second mile means the horse would run .625 of a mile in one minute.
Multiply the result from Step 4 by 60 to get the speed in miles per hour for the horse. A .625 of a mile in a minute reading translates to 37.5mph.