Re: tire sizes

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Posted by Art V. on June 04, 1998 at 13:18:40:

In Reply to: tire sizes posted by Robert E. Weimer Jr. on May 30, 1998 at 11:32:47:

Robert, your speedo is not going to be fast by a measure of MPH, but by a percentage of MPH. Take my case, for instance. My Corolla came with 175/70R-13, and now I've got 195/60R-15 on there. From my calculations, my new tire size means that my car is travelling 7% faster than my speedo reads. Ie: My speedo says I'm going 50, but I'm really going 53.5 MPH.

Here's the calculation for finding the overall diameter of a tire and wheel:

Example 195/60R-15

195=Tire tread width in milimeters
60=Percentage of tire's width to make up wall height (Profile)
15=Diameter of wheel (also inside diameter of tire.) in inches

1. Since about 25.4mm equals 1in., the tire's tread width is 7.67 inches
2. The wall height is 60% of that. . . 4.6 inches
3. Take the wall height, 4.6 and multiply by two (9.2), then add it to the wheel's diameter, 15 inches, to get 24.2 inches.

In my case, the original 175/70R-13 size yields a 22.64in. diameter. So I divide the new size (24.2) by my original size (22.64) and get the percentage size (1.07 or 107%) of the new combination. So I figure my new set is roughly 7% larger, and will show up this way on my speedo.

Try it out. You should get an overall diameter of 25.84 inches for your tires/wheels.

I'll take a look at some websites for some specs on your car. . . what year was it, anyway?


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