More likely, that operating it at the official full speed causes irreparable damage within an hour, and it's traditional for everyone to piss on the radiator to

More likely, that operating it at the official full speed causes irreparable damage within an hour, and it's traditional for everyone to piss on the radiator to help cool it down. How maintainable they are in the field and whether the Army has enough trained mechanics are more important than sexy specs like muzzle velocity and max speed.