The "the" in "in the front" would change the context: it would refer to what the front is the front of, rather than the front itself. In the front of the train transporting cars, in the front of the showroom where cars are on display, in the front of the ship transporting cars across the English Channel. We could, however, say "the car AT the front. The "at" changes the focus to the front point itself, without reference to an outside frame. At the front of the queue, at the front of all the other cars waiting at the traffic lights, and so on. "At the front" can be equivalent to "in front".
I try to think in what context I would say this phrase in either language; most likely when I am following a Ferrari in which case I would say: "the car in front is fast" or "Το μπροστινό αυτοκίνητο είναι γρήγορο". I cannot think of a situation where I would say: The front car is fast
Is it "the leading car" we don't have any indications of that.
"the car in front is fast" THAT IS THE SENTENCE YOU WILL SEE ABOVE AS CORRECT. Why are you suggesting it? Please read the correct sentence at the top of this page.
This sentence: >"the front car is fast " doesn't exist. Why are you mentioning it?