The fastest thoroughbred can run up to 40mph! And for a regular thoroughbred they usually run up to 35-40 mph.
The world record for a Thoroughbred racehorse's average speed over a mile is 43 miles per hour...but their spurts of speed can be slightly faster. Converted to kilometers per hour that equals 69.2km/h.
In the '73 Belmont, it was estimated that Secretariat hit 55 mph down the stretch.