
Speedometer and Odometer Accuracy
Marsha Davis

Mathematics Topic: Algebra, Applied Mathematics, Modeling 
Application Areas: Conversions, Auto Mechanics, 
Prerequisites: Conversions, Excel 

The activities in this PullOut focus
on developing a model to describe
the effect of changing the size of a
vehicle’s tires on the accuracy of
its odometer and speedometer. In
Activity 1 students determine tire
diameter from tiresize labeling; for
example, 265/75R16. This involves
finding a percentage of the tire’s width
(given in millimeters), then converting
the result to inches. After working
through this activity, students should be
able to set up a direct variation between
actual mileage and odometer mileage; the
coefficient of variation depends on the
current tire size compared to the size of
the factoryinstalled tires. For homework,
students apply what they have learned to
the problem of speedometer accuracy. Up
to this point, students work through a
new set of calculations each time a new
tire size is given. Their major task in
Activity 2 is to create an Excel
spreadsheet “conversion calculator” that
calculates tire diameter from the tire size,
actual mileage from odometer mileage,
and actual speed from speedometer
speed. The three changeintiresize scenarios
presented in Activity 2 were taken from
questions posted online by vehicle owners.
Mathematics Prerequisites and Discussion:
Students should have some experience
converting measures of length from one unit
to another. If not, introduce this topic before
students begin working on this PullOut. If
students are unfamiliar with spreadsheets,
show them how to set up a control panel and
enter simple formulas. (See Lesson Notes for
Activity 2.)
Mathematics Prerequisites and Discussion:
Calculators, access to Excel (or another
spreadsheet) for Activity 2.
