I found this in the game files, but editing it does not seem to do anything. Still looking.
public static float CalculateMaximumSpeedMultiplier(int athletics, float baseWeight, float totalEncumbrance)
{
return Math.Min((900f + (float)athletics) / 1000f * (baseWeight * 200f / (baseWeight * 2f + totalEncumbrance)), 1f);
}