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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