Centimetres and millimetres. In past times, it would have been feet and inches.
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
carat
Cubic centimetres or cubic inches.
miles or kilometers
Scientists use Kilometers to measure distance traveled by a car
I would use centimeters.
Kg or tonnes in metric. Pounds or tons in US units.
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.
Meters