As the car is in motion, the dragging force comes into play which would retard it's motion. Here the chief resistance in retardation of motion is the air. This is in consonance with Newtons Third law of motion stating that every action is opposite and equal .
When the car is at rest, the inertia would act on the body of the car and it would offer substantial resistance to keep the car moving.
While it is in motion, some frictional forces are also operative and are expressed as F= mew multiplied by R. Where mew denotes to Coefficient of friction and R is the resistance.
On stationary position of the car, the reaction - force, R will be equal to mg- where m stands for mass of the car and g is the acceleration due to gravity.
The weight of the car can be calculated by the formula P = mf where P is the force and m is the mass and f is the acceleration. Knowing P and f, m can be calculated. If m is expressed in kg and g is in Newton then the unit of force will be in kg - wt.