You missed the point. If you put just "300" on the plates of a 400HP car, no one in the world will know what you're talking about. It wouldn't even get to the point in the thought process to confuse people.
Now if you had "300 HP" on the plate, then maybe. But it would only confuse the people who already know it has 400HP. If I saw a plate that read "300 HP", and knew nothing of the car, I wouldn't be confused and think, "Well, maybe he really has 400HP,... I'm so confused right now." I would logically think it had 300HP, as the amount of real HP it had would be arbitrary to the thought process.... "could it have 200, or 350, or maybe 700 HP????"