Horses are fast. My father owns horses and the rare occasion when I’ve ridden at a full gallop has scared the crap out of me. The horse I ride isn’t even a racehorse – just a slightly stocky Tennessee Walking Horse named Max!
Thoroughbreds are the horse breed used in the major horse races. They are fast. The top speed a thoroughbred has been clocked at is just under 43 mph in a quarter-mile race.
As fast as thoroughbreds are, the American Quarter Horse is faster, having been recorded at a blistering 55 mph. While the Quarter Horse can reach a faster top speed than Thoroughbreds, Quarter Horses usually have shorter legs and strides than Thoroughbreds and usually lose to Thoroughbreds over the longer distances run in the major horse races (1.25 miles). Source.
It seems that racehorses have hit their speed limit – meaning that they aren’t getting any faster. Here’s a chart from FiveThirtyEight.com showing the winning Kentucky Derby Times for the past 113 years:
Why is that? According to Stanford researcher Mark Denny, “there are limits to the ability of either natural or artificial selection to produce ever faster dogs, horses and humans.” In his research Dr. Denny concludes that selective breading, advances in training, and nutrition have peaked out with respect to race horses and thus they aren’t going to get any faster. He reached the same conclusion for Greyhounds.
Interestingly, Dr. Denny’s research postulates that humans can get a little bit faster based on past and current trends. He projects that humans should be able to squeeze out about 2-3% more speed at pretty much every race distance from 100 meter dash to marathon. One reason humans have been getting faster, and may be a bit faster in the future, is that humans have much greater diversity in breeding as compared to racehorses. Racehorses have been narrowly bred, while humans intermingle and breed widely. Greater breeding diversity leads to greater variability which can result in traits that enhance speed.
On a related and interesting point, Greyhounds and Thoroughbreds are pretty close in speed as they both top out over 40 mph. Greyhounds can run a bit faster than Thoroughbreds and have much faster acceleration, but Thoroughbreds tend to be able to run fast longer. Here’s a video showing a race between a racehorse and greyhound (the dog wins by 7/10th sec):
Here’s a compilation of greyhound vs. horse race videos. It appears that it is a “thing” to have horses race dogs.