What Drives a Vehicle?

well mr.. rabbit i don't think your answer is funny nor a satire but if u can u answer me unless like to give a satire

I could say "the person behind the steering wheel" - but I won't.

Torque makes it move, power determines how fast.

A hamster in a treadmill (a tiny fraction of a horsepower) could make a car move, given sufficient gearing, but it wouldn't go very fast .

but can i ask u a doubt sir...... why can't the tractors be made to convert the horse power(capability to perform a heavy work) to a required speed when it is not in use to do such a heavy job

It can be done, but it takes a large range of gear ratios in the transmission system. Many tractors do not contemplate high road speeds, so this is not always done. Tracked vehicles are not good for high speeds, but some wheeled tractors can do around 60 km/h on roadways.

both power and torque is different forms for the same coin according to a simple relation ( power = torque x rev/sec )

If we assume a race car and tractors with the same power .

race car needs high speed with low torque while tractor needs high torque with less speed.
engine generates power , both gear box and differential transfer this power to the other form which is torque

finally no one of them drives the vehicle but the driver does.

Friction. Specifically the friction between the tires and the road, which allows the Earth to push back against the car allowing it to go forward.

Take away that friction and it wouldn't matter how much horsepower the engine produces; the wheels would just spin.