How do different river sprints affect overall avg mph?

model_rides.py

def avg_speed(distance_in_miles, time_in_minutes):
    time_in_hrs = time_in_minutes/60.0
    return float(distance_in_miles)/float(time_in_hrs)

def river_sprint(avg_mph, river_distance=4.5):
    """ 
    Return the time it would take (in minutes) to 
    sprint the river at speed given by avg_mph.
    """
    return (float(river_distance)/float(avg_mph))*60

def model_a_river_sprint(pre_river_distance, pre_river_time_in_minutes, river_sprint_proposed_avg_mph, river_distance=4.5):
    pre_river_sprint = avg_speed(pre_river_distance, pre_river_time_in_minutes)
    complete_ride_distance = pre_river_distance + river_distance
    complete_ride_time = pre_river_time_in_minutes + river_sprint(river_sprint_proposed_avg_mph, river_distance)
    new_average = avg_speed(complete_ride_distance, complete_ride_time)

    print("Average ride speed (pre-river): \t{}".format(pre_river_sprint))
    print("Proposed river sprint avg speed: \t{}".format(river_sprint_proposed_avg_mph))
    print("Average speed including river sprint: \t{}".format(new_average))
    print("+/- difference post river sprint: \t{}".format(new_average-pre_river_sprint))
    print("-----------------------------------------\n")

ride_dist = 20 # This sounds just about right for 2 hills up to Falls Bridge
time_ridden_before_river = 75 # in minutes!

for speed in range(20, 26):
    model_a_river_sprint(ride_dist, time_ridden_before_river, speed)