How To Find Tire Diameter

Tire diameter of the metric or p-metric tire can be calculated by using its size nomenclature. The section width & aspect ratio are molded into the sidewall. The section height (SH) of a tire is the section width multiplied by the aspect ratio. The overall diameter (OD) includes two section heights converted to inches, plus one rim diameter.

OD = (2x SH) + Rim Diameter

This formula is valid for all metric & P-metric tires.

SPEEDOMETER ERROR

Speedometer data from vehicles is used in many applications besides providing vehicle speed information to the driver. Engine computers, cruise control, transmission shifting & some anti-lock brake systems are affected by speed input.

It is important that the speedometer accurately indicate the vehicle's true speed. The easiest way to check speedometer accuracy is to time how long it takes to drive one mile. Most limited-access highways have mile markers that help in a speedometer check. Sometimes bridge & ramp locations affect mile marker positioning. Speeds should be checked during periods of light traffic so that constant speeds can be maintained. On vehicles equipped with cruise control, the task is easier than on vehicles not so equipped.

Speed should be checked for several consecutive miles to obtain consistent readings. If consistent times cannot be obtained, try another set of mile markers. Once the vehicle's indicated speed, & the average time it takes to travel one mile are known, the calculations are simple. Sixty miles per hour equates to one mile per minute. If it takes 60 seconds to travel one mile, the speed is 60 miles per hour.

Here are other times & speeds:

For other times, divide 3600 by the time in seconds to get the average speed in miles per hour.

MPH = 3600 / TIME (sec)

To calculate speedometer error, take the true vehicle speed as calculated from a timed mile, subtract the indicated speed as shown on the speedometer & divide the difference by the true speed.

ERROR = INDICATED SPEED - TRUE SPEED X100
TRUE SPEED

To ensure that the tires you install on your vehicle will not greatly alter the speedometer or odometer readings, check the Original Equipment tire's outside diameter. Then verify that the outside diameter of the replacement tire is within 3%. Your Dunlop dealer can assist you in making the right choice.

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