Teaching Physics with the Physics Suite

Edward F. Redish

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Fuel Efficiency

In America, we measure fuel efficiency of our cars by citing the number of miles you can drive on one gallon of gas (mi/gal). In Europe, the same information is given by quoting how many liters of gas it takes to go 100 km (li/100 km).

  1. My current car gets 21 mi/gal in highway travel. What number (in li/100 km) should I give to my Swedish friend so that he can compare it to his Volvo?
  2. The car I drove in England last summer needed 6 liters of gas to go 100 km. How many mi/gal did it get?
  3. If my car has a fuel efficiency, f, in miles/gallon, what is its European efficiency, e, in liters/100 km? (Write an equation that would permit you to make an easy conversion from one to the other or back.)
  4. The fact that the US figure, f, and the European figure, e, are inverses mean that they behave differently. Suppose you compare three cars: an SUV that gets 10 mi/gal, a compact car that gets 20 mi/gal, and a hybrid that gets 35 mi/gal. Make a guess as to which transition will save you more money: changing from the SUV to the compact or changing from the compact to the hybrid? Assuming that gas costs $4/gal and you drive 1000 mi/month, which transition would save you more money? Calculate the European values for these three cars. Which figure, e or f gives you a better idea of which shift will save you more money? Explain why.

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Page last modified September 4, 2008: G04