I went downtown in a car/on a bus.
If you went "on a car", it would mean that you were physically on top of the car, on the outside, so you wouldn �t use that expression. We also don �t say "in a bus".
As libertybelle said, you can also go by car or by bus.
We use "in" with small vehicles that hold few people:
in a car, in a taxi, in a van, in a boat (if it �s a small one)
We use "on" with large vehicles that hold many people:
on a bus, on a ship, on a yacht, on a train.
Generally, if you can comfortably walk around in it, use "on". Otherwise, use "in".
Of course, we also use "on" with something that we are physically on, such as a bicycle, skateboard, etc.
Bruce
