Neither, I'd rather measure a car as about 13 ft 2 in long.
I'd recommend that you ought to measure a car's length as about 4 m in length as a car 4 km long would be about 2.5 miles long.
As a guide: the longest trains average about 2 km (1.25 miles) long with the longest in the US limited by air brake capability to about 3.7 km long.
The longest ever train consisted of 682 loaded wagons and 8 locomotives evenly placed throughout the train for a total length of 7.353 km (4.568 miles). It was all controlled by one driver.
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