An unmanned Rocket Sled at Holloman Air Force Base on April 30th 2003 reached 6453 MPH!
Some details from the Wiki:
“It was the final stage of a four-stage sled train, which included the sled, a Super Roadrunner rocket motor, and a 192-pound payload on top. The sled train delivered payload into a target at a velocity of 9465 feet per second, or 6453 mph (2885 m/s), or Mach 8.5.”
Hawk (Feels the need for SPEEEEED!)