Most introductory physics students have seen vehicles with nonstandard wheel diameters; some may themselves drive “low‐rider” cars or “big‐wheel” pickup trucks. But how does changing wheel diameter affect speedometer readout for a given speed? Deriving the answer can be followed readily by students who have been introduced to rotation, and it makes a good illustration of how reasoning in physics can lead to a result that is useful outside the classroom.
REFERENCES
1.
The interpretation of the tire designation, and also Fig. 2, are adapted from The Tire Rack Performance Source; www.tir‐erack.com/tires/tiretech/techpage.jsp?techid=46.
2.
The examples used here illustrate a common tire designation system called “P‐metric.” Other systems may be encountered, for example, on truck tires or on older vehicle tires. See the website referenced above for descriptions of other systems.
3.
www.csgnetwork.com/speedocalibcalc.html.
4.
Some vehicles with nonstandard wheels may have had speedometer adjustments done to compensate for the change. In such cases, of course, the reasoning here would not apply.
5.
The procedure used here for finding the effective wheel diameter was suggested by an anonymous peer reviewer.
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© 2010 American Association of Physics Teachers.
2010
American Association of Physics Teachers
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