To measure a car's length, I would try yards or feet. Maybe both. It mostly depends on the legnth of a car. A stretch limo would use yards while a VW Bug would probably be feet. For an average car, though, yards.
Meter
mm, cm or m.
meter A kilometer is 1000 meters.
what is the most suitable unit to measure the length of a car key
miles or kilometers
I would use centimeters.
feet The meter (m) is the international unit (in SI) for length.
The metre, the yard or the foot.
Centimetres and millimetres. In past times, it would have been feet and inches.
carat
meters
You would measure the length of a car in metres. An average car is about 4m