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.
The revolution of a tire is equivalent to the circumference of the tire.
The Circumference of the tire is equal to PI times the diameter.
1 Mile = 5280 feet.
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
For every 560.51 revolutions of the tire, the odometer will register 1 mile.
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% = 0.64%
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.