As an english speaker I think "Er passt in das Auto" means "He fits into the car".
It seems more likely. Since "in" can get the acusative when there is movement to a destination....
Yes, but I think that "into" is British English. This platform here usually uses American English.
(I am a German speaker)
"Into" is most certainly used in America. It often has a connotation of motion toward(s).
Are you a native American English speaker or a native German speaker? I ask because if you are a native German speaker, can you tell me if the sentence means a) "He is capable of fitting in the car" (i.e. he is is not too tall/fat) or b) right now he is getting into the car or c) ... something else?
I am confused. I was under the impression that in+accusative indicates the direction of motion (into) while in+accusative indicates location (inside). He either fits or he doesn't inside the car. Why is this direction and not location?
Also it is about the verb, here "passen". It is indeed related to "to pass" which had originately the idea of A going past B. But this idea changed a bit to B letting A past, or better A fullfilling the criteria for being let past. In this case, A (he) has the right size to come past the obstructions set by B (the car). But you still have the grammatical reminder that the idea is him moving past this obstacle into the car, hence the accusative. "To fit" has a different heritage without a German brother so it works differently.
Anyway, the difference should not be "direction" vs "location" but "target of action" (accusative) vs "location of action" (dative). Your wording is not wrong but it might confuse in such contexts like here. The car is definitely his target for fitting inside and not where he fits around in (whatever that should mean).
I hope I could be of help.
The sentence or thought makes sense in both languages, if properly translated from one to the other. The sentence means he fits in the car. He is not too tall or fat, etc. Consequently, English-speakers learning German expect the German translation to use the dative "in dem Auto," because no action is happening. But the German idea of fitting in the car is that he is ABLE to fit INTO the car, the ACTION is possible, and thus Germans use the accusative: "in das Auto."