You need to include the rest of your sources (car.cpp, motor.cpp and tires.cpp) in the build in some way.
You can either add them along with main.cpp in the executable directly:
set(MySources sources/car.cpp sources/motor.cpp sources/tires.cpp main.cpp)
add_executable(${EXECUTABLE_NAME} ${MySources})
or you can make these into a library and link that:
set(MyLibSources sources/car.cpp sources/motor.cpp sources/tires.cpp)
add_library(MyLib ${MyLibSources})
add_executable(${EXECUTABLE_NAME} main.cpp)
...
target_link_libraries(${EXECUTABLE_NAME} MyLib ${SFML_LIBRARIES})
A couple of other points to note:
You should avoid the use of link_directories
if possible (its own documentation discourages its use), and it's often helpful to include the headers in the list of files added via add_executable
or add_library
since these then show up in IDEs like MS Visual Studio.