Effect of tire size on odometer readings

Written by George Mastros
Orbit Software, Inc.
November 17, 2004

Assumption:

• The odometer “calculates” the distance based on the number of revolutions for the tire.
• The diameter of a bus tire is 36 Inches.
• Tire treads are worn evenly over the entire surface of the road. Therefore; 1/8” tread wear amounts to 2/8” reduction in diameter.

Observations:

• The revolution of a tire is equivalent to the circumference of the tire.

Fact:

• The Circumference of the tire is equal to PI times the diameter.
• 1 Mile = 5280 feet.

Calculations:

• The Circumference of a 36 inch tire is PI * 36 = 113.1 Inches or 9.42 feet
• The Circumference of a 35 6/8” inch tire is PI * 35.75 = 112.3 inches or 9.36 feet
• Calculate the revolutions required to travel 1 mile (5280 feet). 5280 / 9.42 = 560.51 revolutions

Intermediate Conclusion:

• For every 560.51 revolutions of the tire, the odometer will register 1 mile.

Further Calculations:

• In 560.51 revolutions, a 35 6/8” tire will travel… 560.51 * 9.36 or 5246.37 feet.
• For each 560.51 revolutions, the odometer will increment by 1 mile even though the vehicle has only traveled 5246 feet. Therefore, a vehicle with tread wear will report more miles driven by the odometer.
• The difference in distance is 33.63 feet for each mile driven. Actually traveling 157 miles will cause the odometer to be off by 1 mile.
• The percentage of error is… (1 - 5246.37 / 5280) * 100% = 6.4%

Conclusions:

• 1/8” tread wear on a 36-inch tire reduces the circumference by 0.8 inches.
• Over the length of 1 mile, this adds up to 33.6 feet.
• After “actually” traveling 157 miles, the odometer will be “off” by 1 mile. It will read 158 miles.
• Smaller diameter tires will effect odometer reading more than larger tires.