# The Naked Scientists Forum

### Author Topic: How efficient is my car?  (Read 6547 times)

#### RD

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##### How efficient is my car?
« Reply #25 on: 25/08/2009 11:46:34 »
Efficiency: the percentage of the chemical energy in the fuel which is converted into kinetic energy.

kinetic energy = 1/2 x (mass of vehicle & load) x (speed of the vehicle)2 .

Strap a refrigerator to the roof of your car and the increased air resistance would reduce its MPG to that of your truck.
The accepted definition includes "useful work", not just KE. After all, going at 70mph, mile after mile, involves no increase in KE but involves steady expenditure of work.

expenditure of work primarily against air resistance (when travelling at a constant speed).

By stating KE = 1/2 m v 2  I was trying to point out that more energy would be would be required to attain the same speed with additional mass (gravel), but the efficiency of the engine would be the same.  Loading the vehicle does not make it less efficient but does mean that more energy is required to attain the same speed as when unloaded.

#### Geezer

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##### How efficient is my car?
« Reply #26 on: 25/08/2009 17:53:32 »

Without asking a lot of questions like "efficient at doing exactly what, big guy?", it's not possible to answer the question - although I did leave a clue about asking questions.

The measure of "efficiency" might not even have anything to do with fuel consumption (a red herring, I confess). For example, we might only be interested in the efficient use of the driver's time. Clearly the truck is much more efficient in that respect if we have to lug 1500 pounds of gravel somewhere.

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##### How efficient is my car?
« Reply #26 on: 25/08/2009 17:53:32 »