Question

When a speeding truck hits a stationary car, the car is deformed and heat is generated. What can you say about the kinetic energy of the system after the collision?

Answer 1

I can say that the kinetic energy after the collision is less than it was before the collision.  I can say this with confidence because you've said that some energy was used to deform the car, plus there was energy lost from the system in the form of heat.