The very earliest of tyre designs were called Type I and were referred to only by their outside diameter eg 27" which is a common fitment to aircraft such as Harvards or P51 Mustangs.
General Aviation tyres are generally Type III designs and include the most widely used sizes such as 5.00-5 and 6.00-6. These measurements refer to the section width of a tyre and also the rim ledge diameter (or the hole in the middle) hence a 6.00-6 is 6 inches wide and sits on 6 inch wheel.
Three Part Name Sizes cover most of today’s designs and typically describe the Outside Diameter, Section Width and Rim Ledge Diameter. Eg, 15x6.00-6, the tyre is 15 inches tall, 6 inches wide and sits on a 6 inch wheel. Some tyres in this classification are preceded with an ‘H’ which identifies the tyre as having a higher percentage deflection.
Metric sizes are the same as three part name sizes but the Overall Diameter and Section Width are replaced with millimeters but the rim sizes remains in inches, eg 380x150-5, it’s imperial equivalent is a 15x6.00-5.
Radial tyres are also the same as three part name sizes but the dash preceding the rim diameter is replaced with the letter ‘R’ eg 32x8.8R16
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