What makes the physics car react?
Both the top-down car and the side-view car "react" to the same things
and its important to keep in mind.
The acceleration is based on wheel friction and acceleration (see properties). For a standard 2WD physics car, the acceleration is based
on the friction of the two front wheels. The less friction the less will
the acceleration be. This is then scaled with the acceleration value set
in the properties settings.
De-acceleration means basicly if you don't accelerate or brake with the car. As with the acceleration, the de-acceleration is also based on wheel friction, but its calculated by all wheels. Another variable
that plays a very big part here, is the linear damping of the car body.
The linear damping should be seen as wind resistance or similar, and should never be 0 (zero) because then the car will roll on forever.
As said above, braking is also based on the wheel friction and the
"brake/reverse" property in the settings.
Turning with a car (only applicable for the top-down car physics),
is based on a max steering variable in the property settings and
a speed variable, also in the property settings. The Max steering
means how "much" you can turn a wheel, and the speed means how fast
you can turn the wheel.