k you have gravity, the cars Y value will decrease every loop... EVEN if on the ground, then if it goes below ground height, update the Y value to the current Ground level, simple

. This way when the Y level is higher than the Ground level it decreases until the Car hits the ground again =D Hope that helps mate.
arced motion? Well now.. you could have it if the car is going faster the long air time it can have =). Or you can have some jump code, meaning it calculates the speed + angle of the car when it leaves the ground. Then you can create the code to allow th car to go up in the air depending on that calculation, BUT remember to have the car also head back to earth

don't want it to keep going up to it see's the stars hehe