Speedometer Error Related to Tire Diameter

Description

The speed displayed by the speedometer is derived by counting the wheel rotation for a given time period, then using the tire circumference to determine the distance. A change in the tire diameter can cause the speedometer to

Parameters:

(1) T/A_W for standard tire

(2) T/A_W for new tire

T is nominal section width in mm

A is tire aspect ratio in percent

"_" is the optional letter indicating the construction of the fabric carcass

W is wheel diameter in inches

tire diameter in inches =

= ((T * A) /1270) + W

percentage error in speedometer reading =

= 100 * (1 - ((new diameter) / (standard diameter)))

speedometer reading =

= (true speed) * (100 + (percentage error))

true speed =

= (speedometer reading) / (100 + (percentage error))

Tire Size	Speedometer	Actual Speed
smaller	overstimates	slower
larger	underestimates	faster

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