Thursday, August 24, 2006

Tyre Sizes Explained


By David Lovejoy, as extracted from Difflock Web Magazine.

Many of us often curious about differences in how people measure a tyre size. Some says their tyre size is 31/10.5x15, some says 205x75x15. Also, why there is an increase in gearing when tyre is changed from a 30" diameter to 31" diameter.

Tyre sizes are basically measured by the outside diameter, width and diameter of wheel rim. The measurement itself generally using 3 known universal systems: Imperial, American, and Metric.

Explained:

  • Imperial (7.5x16), meaning a tyre 7.5 inches wide fitted to be fitted on a 16 inches wheel.
  • American (31/10.5x15), meaning 31 inches diameter, 10.5 inches wide to be fitted on a 15 inches wheel
  • Metric (205/75x16), meaning 205 milimeters wide with an aspect ratio (sidewall height) of 75% of the width, to be fitted on a 16 inches wheel.
So, why gearing should be considered when changing tyre sizes? Very basic, it is because you change the rolling circumference of the tyre. Circumference changes as soon as you increase the diameter (or height) of the tyre.

Explained:

Say you are changing your vehicle's 205/75x16 tyre to a 31/10.5x15. The 31/10.5x15 is 10.5 inches wide, whereas the 205/75x16 is a shade over 8 inches wide (205mm). The 31/10.5x15 is approximately 31 inches high. To work out the height of the 205/75x16 follow this sum:

First find out 75% of 205mm (to get the aspect ratio), this is 153.75mm, double this figure (you have two aspect ratios) to get 307.5mm, then divide by 25 (25mm is 1 inch) which will give you 12.3 inches. Add your 16 inches from the wheel diameter to the 12.3 inches of total aspect ratio. This will give you 28.3 inches of total height or diameter.

To get the percentage you have moved up from the original gearing divide the diameter of the new tyre with that of the old. You will find that you have increased the gearing by almost 10%. This isn't a great deal if you have a V8 but it makes a hell of a difference to your average tyred 2.5 diesel engine! To obtain the same power/ torque, you should increase your final gear ratio by at least of that 10% increase in gearing. So if your OEM final gear ratio is 3.5, you should replace it with a 3.85 ratio to obtain the same power like when using the original 205/75x16 tyre.

Have fun trying!