At a gallop, the fastest of all gaits, an average horse can run about 25 to 30 miles per hour. However, many factors depend on the speed of the horse.
The Thoroughbred, one of the fastest breeds, tops out at about 40 miles per hour. The fastest horse ever recorded ran 43.26 miles per hour.
The Quarter Horse can reaches a top speed of about 55 miles per hour. As the name suggests, the quarter horse runs a quarter-mile race, and the faster horses finish in about 20 seconds.
In a quarter-mile race, a Quarter Horse will almost always win against a Thoroughbred. In a longer race, a Thoroughbred will almost always win against a Quater Horse.
One of the key requirements to survive out in the open was the ability to outrun predators - so it is perhaps not surprising that the horse developed into a finely tuned running machine.
Obviously every horse is unique, and the speed difference between, say, a Shire horse and a Thoroughbred is massive. However, by comparing a number of different sources, both historical and scientific, we can determine the average speed of the fastest type of horse.
According to the World Book Encyclopaedia's 'Horse' entry, "… the thoroughbred can run … a mile (1.6 kilometres) in about 1 and 1/2 minutes" or 17.78 meters per second. Race coverage from sporting sites such as ESPN tends to support this - with the high end of race performances topping out at between 16.5 and 17.5 meters per second - or around 38 miles per hour.
Not surprising then, that being a professional jockey is more dangerous than being a professional skydiver.