We have a 4x6 differential (tank) drive system. There are 2 cims/side mated to supershifters. The maximum speed in a straight line on a linoleum floor (observed) is 18ft/sec, but with the shifters our track speed (with traffic) is still a good haul at about 12.75ft/sec.
Don't believe me for speed? Just watch our rear data center. It displays the robot's overall vector velocity in ft/sec by default in teleoperated mode.
-q