Setting Sampling Preferences describes how you can use the Sampling tab of the Run Preferences dialog to use the same sequence of random numbers for each simulation.
If you use Define, and then Define Assumption or the Define Assumption toolbar button to define assumptions, the same sequence of random numbers is used for each simulation, even if you switch from Extreme speed to Normal speed or back to Extreme speed. If you use the probability functions to define assumptions, one sequence of random numbers is used for Extreme speed and a different sequence is used for Normal speed.