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 different 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

Re: Please help with this "find this equation" question

What does "rim size" mean? The circumference?

Re: Please help with this "find this equation" question

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 d_{1}) and with the actual tires (call it d_{2}).

Suppose the tires make n revolutions in time t. Then the displayed speed is $\displaystyle v_1=n(\pi d_1)/t$, while the actual speed is $\displaystyle v_2=n(\pi d_2)/t$. Therefore $\displaystyle v_2/v_1=d_2/d_1$. Use this equation to express the true speed $\displaystyle v_2$ through the displayed $\displaystyle v_1$.

Re: Please help with this "find this equation" question