Merriam webster made you suppose automobile is the normal word for car. This is a weak point of dictionaries that don’t hint at the main variant. You can see the normal variant of car on google ngram which can be easily found by Google. Automobile is a relatively seldom used variant in written language. Is the length of a English dictionary too long for the current incarnations of all the spoken languages?

Do

have a look at synonyms for car at Merriam Webster or The Free Dictionary.

The terms are roughly synonymous but do have distinct uses that set them apart from each other as well. Two point to respective classes of vehicles, however the complete range of vehicles each term form separate but overlapping sets.

Can car refers to objects that aren’t automobiles (e.g., railway carriages) Full Definition of

Car 1 : –

A vehicle moving on wheels: as a archaic

• : carriage, chariot b: a

• vehicle designed to move on rails (as of a railroad) c : automobile

• 2 : the

passenger compartment of an elevator 3 : the

part of an airship or balloon that carries the passengers and cargo source : Merriam

Webster. com

Secondly automobile can be used as a general classifying term that is inclusive of vehicles other than cars (and not inclusive of non-wheeled, non-self-propelling instances of “car”, such as airship motor car etc) as per:

source : http://what-when-how.com “com/automobile/general-classification-of-automobiles/….”