In the maneuver commands, you can use the RealTimeExpressions function max().

The max-function returns the maximum value of the given arguments.
The best way to get a maximum value of one quantity for the whole simulation is the following approach. Add the following commands in the Global Maneuver commands:

first() ? Qu::max_speed = 0
max_speed = max(Car.v,max_speed)

The variable "max_speed" is there to save the maximum value.
Then each simulation cycle we compare the current speed "Car.v" with the maximum value.
Which of both is bigger is saved as the new maximum speed.

You can check this screenshot to see how the values change: