At high speed(say 120km/h), why wont the engine consume the same amount of petrol per unit distance,

 at lower speeds[say at 40km/h)

When I move a block from Pt A to B, the work done is same whether it is moved fast or slow, right? So whats wrong with cars

the power of the engine is F.v

F is the opposive force experienced by the car neglecting the drag forces the external force acting is frictional force which is constant for any speed

so at high speed v will be higher and hence the work done per unit time by the engine will be higher and hence more fuel is consumed

@v.sabarish 

Your answer makes sense. Guess its rit. Thanks

im satisfied with pratabh higher speeds more drag force to overcome

so more consumption

but tell me why there is an optimum speed of about 50kmph recommended for best fuel effc.