What is the car\'s efficiency? Driving on asphalt roads entails very little roll

Question

What is the car's efficiency?

Driving on asphalt roads entails very little rolling resistance, so most of the energy of the engine goes to overcoming air resistance. But driving slowly in dry sand is another story. If a 1500 kgcar is driven in sand at 5.9 m/s , the coefficient of rolling friction is 0.06. In this case, nearly all of the energy that the car uses to move goes to overcoming rolling friction, so you can ignore air drag in this problem.

If the car gets 15 miles per gallon when driving on sand, what is the car's efficiency? One gallon of gasoline contains 1.4×108 J of chemical energy, one mile is 1609 m.

I calculated the propulsion force to be 882N & Power to be 5200W.

Explanation / Answer

15 miles = 24140 m
1 gallon of gasoline contains 1.4×10 J.
So moving a distance of 24140m requires gasoline containing 1.4×10 J
Therefore moving a distance of 1m requires gasoline containing 1.4×10/24140 = 5800 J
Overcoming rolling resitance for 1m requires (useful) work = force x distance = 1000x1 = 1000J
So 5800J (in the gasoline) provides 1000J (overcoming rolling resistance) of useful work for each metre moved.
Efficiency = useful work/total energy supplied
= 1000/5800
= 0.17 (=17%)

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