As the story goes the Swiss once tuned one to do 140km/h, made a break test, and had to send the crew to hospital. Without messing with the transmission to get higher ratios, well-maintained, optimally tensioned tracks etc. the maximum is about 90, more typically 80, the official maximum is 70. Soldiers are generally told to keep it under 60. Don't underestimate the centrifugal force of the tracks they're getting hard to control at high speeds.