A toy car rolls down a ramp, even absence of non-conservative forces, it's not possible for all of it's potential energy to convert into kinetic energy.
What is moving, within the car-system itself, that is also gaining energy
in order for this part of the car to move? How is this responsible for
consuming some of the mechanical energy?

The magnitude of the amount of energy released by burning a fuel source, measured in energy per unit mass, is called its fuel value. note that the fuel value is the negative of the isobaric specific heat of combustion for the fuel. if all the energy obtained from burning 1.23 pounds of butane with a fuel value of 10.85 kcal/g is used to heat 128.0 kg of water at an initial temperature of 18.3 °c, what is the final temperature? note that 1 lb = 453.6 g.