If you know so much then you could have estimated his speed from the damage. After all, you said you knew it wasn’t 40 mph. Either you know what you’re talking about or you don’t.
Take a look at this video, a 40mph partial overlap crash of a 2016 Dodge Challenger into a barrier.
The Challenger at the crime scene didn't look like that did it? And yes, I know that the barrier test does more damage than a collision with another vehicle which slides on the pavement, and the damage pattern is somewhat different if the two vehicles are fully aligned. So here is one where the car is fully aligned with the barrier:
Notice how much more damage the Charger has?
Lesson #2. The accurate way to determine the collision speed is to measure the motion of the car in successive frames of the video. That's going to generate very accurate data. Your second homework assignment is to do that. Get google map data of the dimensions of the street, look at the video, and figure out the velocity by measuring the distance the car moves in a fixed interval of time. Basic physics, if you remember that. Post your results so we can all grade you.
Lesson #3. The collision with the stopped cars pushed them forwards, with tragic results, and as you can see in the videos caused moderate damage to the first car, very little to the van, and slight to moderate damage to the Challenger. your homework assignment is to look at the damage to the car hit by the Challenger. And then compare it to the damage which resulted in the following videos:
Crash at 10 mph: Notice amount of damage to both vehicles
Crash at 20 mph: Notice amount of damage to rear structure, window, etc.
When you are done your homework, post your results, and if you really think that the Challenger was going 40mph when it hit the other cars provide some evidence to support your belief.