We don't have synonyms for motor cars yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are motorcar, motor car and motorcars.
(n) :
Alternative form of motorcar (“automobile”). [(dated) An enclosed passenger motor vehicle: a car (automobile).]
(n) :
(rail transport, US) Alternative form of motorcar (“self-propelled railway vehicle”). [(dated) An enclosed passenger motor vehicle: a car (automobile).]
(n) :
Alternative form of motorcar (“automobile”). [(dated) An enclosed passenger motor vehicle: a car (automobile).]
(n) :
(rail transport, US) Alternative form of motorcar (“self-propelled railway vehicle”). [(dated) An enclosed passenger motor vehicle: a car (automobile).]