The highest race horse speed ever recorded was 43.97 mph (70.76 km/h).

The highest race speed recorded over two furlongs is 70.76 km/h (43.97 mph) and was achieved by Winning Brew trained by Francis Vitale (United States), at the Penn National Race Course, Grantville, Pennsylvania, United States, on 14 May 2008. Winning Brew covered the quarter-mile (402 m, 2 furlongs) in 20.57 sec.

Source: guinnessworldrecords.com/records…