Um I suspect that's the official speed the actual speed would be classified I know people who where passed by chieftains on Salisbury plain going >50mph in the 70's
With the M1A2 engine governor removed, speeds of around 60 mph (97 km/h) are possible on an improved surface; however, damage to the drivetrain (especially to the tracks) and an increased risk of injuries to the crew can occur at speeds above 45 mph (72 km/h).