Thoroughbreds can sustain a speed of 40 miles per hour during races. Although they are very swift, the quarter horse breed is actually substantially faster, with a maximum speed of nearly 55 miles per hour.
Thoroughbred horses have been bred for speed for generations in order to win horse races. The competition has given thoroughbreds an average top speed close to 40 miles per hour, with few outliers in either direction. The fastest thoroughbred in the world, Winning Brew, clocked in at 43.97 miles per hour at Penn National Race Course in 2008. The fastest thoroughbred over the mile and a half, Hawkster, ran it at 37.82 miles per hour in 1989.
Although thoroughbreds can hit their maximum speed over the course of a race, they do not run at top speed throughout. Unlike quarter horses, which accelerate throughout a race, thoroughbreds tend to reach their maximum speed in the middle of a race and slow toward the end.