By definition,
Momentum = Mass * Velocity
Let v = the velocity of the truck, m/s
The mass of the truck is 36,287 kg.
The momentum is 907,175 (kg-m)/s.
Therefore
907,175 (kg-m)/s = (36287 kg)*(v m/s)
v = 907175/36287 = 25 m/s
Answer: 25 m/s
Mechanical Energy transforms into Thermal due to the moving parts rubbing on eachother creating heat and friction.
Average speed is defined by the following formula

here
D = total distance that an object move from its initial position to final position
t = total time of the motion
so here we will say that there is no such relation between initial or final speed or we can say maximum or minimum speed of object with average speed of object.
We only need to find the total distance and total time of motion in order to find the average speed
here we can see many examples like let say an object moves with speed v1 for time t1 and then with speed v2 for time t2 then here average speed is given as

since we know that distance covered is product of speed and time
that's why we used above equation for finding total distance
now the average speed will be

so this is how we can find the average speed for above motion
so average speed is always between maximum and minimum speed any value in-between.
It is neither the maximum value nor it is minimum value