# Please help with this "find this equation" question

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• Apr 12th 2013, 05:19 AM
mathkid182
Please help with this "find this equation" question
The Holden is meant to have 195/65R15 tyres.

However when I purchased my Holden it had 195/60R15 tyres.
Because the rolling diameteris diff
erent this has affected the speedometer and odometer accuracy. Determine the rolling diameter of each tyre and determine the equation thatrelates the true speed with the displayed speed.

FYI: The tyre size W/P R I means the tyre has a width of W in millimetres(the width of the part of the tyre tread), it has a profile or aspect ratio ofP percent (the distance from the rim to the tread divided by the width ex-pressed as a percentage) and an rim size of I inches (note 1 inch equals 2.54cm).

any help would be great thanks very much! so sure how to even start
• Apr 12th 2013, 05:27 AM
emakarov
Re: Please help with this "find this equation" question
What does "rim size" mean? The circumference?
• Apr 12th 2013, 05:28 AM
mathkid182
Re: Please help with this "find this equation" question
yes i believe so
• Apr 12th 2013, 06:25 AM
emakarov
Re: Please help with this "find this equation" question
Quote:

Originally Posted by emakarov
What does "rim size" mean? The circumference?

Quote:

Originally Posted by mathkid182
yes i believe so

In fact, it seems to be the diameter of the rim, as described in Wikipedia and here (search for "Tire and Wheel Diameter").

So, you have the rim diameter d, the tire sidewall height h and the tire width w. The three numbers in the tire code specify w, h / w, and d, respectively. First convert d into millimeters (or millimetres, as your Pommy textbook calls it (Smile)). Then find h from w and h / w. The rolling diameter of the wheel (rim plus tires) is d + 2h. Find the diameter of the wheels with correct tires (call it d1) and with the actual tires (call it d2).

Suppose the tires make n revolutions in time t. Then the displayed speed is $v_1=n(\pi d_1)/t$, while the actual speed is $v_2=n(\pi d_2)/t$. Therefore $v_2/v_1=d_2/d_1$. Use this equation to express the true speed $v_2$ through the displayed $v_1$.
• Apr 21st 2013, 05:59 AM
BenBrawn
Re: Please help with this "find this equation" question
It's Aussie mate. :P